feat: support sharable configuration

Adds the options `extends`, which can be defined via configuration file or CLI arguments to a single path or an array of paths of shareable configuration.
A shareable configuration is a file or a module that can be loaded with `require`.
Options is defined by merging in the following order of priority:
- CLI/API
- Configuration file
- Shareable configuration (from right to left)

Options set in a shareable configuration can be unset by setting it to `null` or `undefined` in the main configuration file. If a default value applies to this property it will be used.

test/plugins/plugins.test.js

@@ -1,15 +1,26 @@
+import path from 'path';
 import test from 'ava';
+import {copy, outputFile} from 'fs-extra';
 import {stub} from 'sinon';
+import tempy from 'tempy';
 import getPlugins from '../../lib/plugins';
 
+// Save the current working diretory
+const cwd = process.cwd();
+
 test.beforeEach(t => {
   // Stub the logger functions
   t.context.log = stub();
   t.context.logger = {log: t.context.log};
 });
 
+test.afterEach.always(() => {
+  // Restore the current working directory
+  process.chdir(cwd);
+});
+
 test('Export default plugins', t => {
-  const plugins = getPlugins({}, t.context.logger);
+  const plugins = getPlugins({}, {}, t.context.logger);
 
   // Verify the module returns a function for each plugin
   t.is(typeof plugins.verifyConditions, 'function');
@@ -28,6 +39,62 @@ test('Export plugins based on config', t => {
       analyzeCommits: {path: './test/fixtures/plugin-noop'},
       verifyRelease: () => {},
     },
+    {},
+    t.context.logger
+  );
+
+  // Verify the module returns a function for each plugin
+  t.is(typeof plugins.verifyConditions, 'function');
+  t.is(typeof plugins.getLastRelease, 'function');
+  t.is(typeof plugins.analyzeCommits, 'function');
+  t.is(typeof plugins.verifyRelease, 'function');
+  t.is(typeof plugins.generateNotes, 'function');
+  t.is(typeof plugins.publish, 'function');
+});
+
+test.serial('Export plugins loaded from the dependency of a shareable config module', async t => {
+  const temp = tempy.directory();
+  await copy(
+    './test/fixtures/plugin-noop.js',
+    path.join(temp, 'node_modules/shareable-config/node_modules/custom-plugin/index.js')
+  );
+  await outputFile(path.join(temp, 'node_modules/shareable-config/index.js'), '');
+  process.chdir(temp);
+
+  const plugins = getPlugins(
+    {
+      verifyConditions: ['custom-plugin', {path: 'custom-plugin'}],
+      getLastRelease: 'custom-plugin',
+      analyzeCommits: {path: 'custom-plugin'},
+      verifyRelease: () => {},
+    },
+    {'custom-plugin': 'shareable-config'},
+    t.context.logger
+  );
+
+  // Verify the module returns a function for each plugin
+  t.is(typeof plugins.verifyConditions, 'function');
+  t.is(typeof plugins.getLastRelease, 'function');
+  t.is(typeof plugins.analyzeCommits, 'function');
+  t.is(typeof plugins.verifyRelease, 'function');
+  t.is(typeof plugins.generateNotes, 'function');
+  t.is(typeof plugins.publish, 'function');
+});
+
+test.serial('Export plugins loaded from the dependency of a shareable config file', async t => {
+  const temp = tempy.directory();
+  await copy('./test/fixtures/plugin-noop.js', path.join(temp, 'plugin/plugin-noop.js'));
+  await outputFile(path.join(temp, 'shareable-config.js'), '');
+  process.chdir(temp);
+
+  const plugins = getPlugins(
+    {
+      verifyConditions: ['./plugin/plugin-noop', {path: './plugin/plugin-noop'}],
+      getLastRelease: './plugin/plugin-noop',
+      analyzeCommits: {path: './plugin/plugin-noop'},
+      verifyRelease: () => {},
+    },
+    {'./plugin/plugin-noop': './shareable-config.js'},
     t.context.logger
   );
 
@@ -41,7 +108,7 @@ test('Export plugins based on config', t => {
 });
 
 test('Use default when only options are passed for a single plugin', t => {
-  const plugins = getPlugins({getLastRelease: {}, analyzeCommits: {}}, t.context.logger);
+  const plugins = getPlugins({getLastRelease: {}, analyzeCommits: {}}, {}, t.context.logger);
 
   // Verify the module returns a function for each plugin
   t.is(typeof plugins.getLastRelease, 'function');
@@ -55,6 +122,7 @@ test('Merge global options with plugin options', async t => {
       otherOpt: 'globally-defined',
       getLastRelease: {path: './test/fixtures/plugin-result-config', localOpt: 'local', otherOpt: 'locally-defined'},
     },
+    {},
     t.context.logger
   );
 
@@ -64,8 +132,10 @@ test('Merge global options with plugin options', async t => {
 });
 
 test('Throw an error if plugin configuration is missing a path for plugin pipeline', t => {
-  const error = t.throws(() => getPlugins({verifyConditions: {}}, t.context.logger), Error);
+  const error = t.throws(() => getPlugins({verifyConditions: {}}, {}, t.context.logger));
 
+  t.is(error.name, 'SemanticReleaseError');
+  t.is(error.code, 'EPLUGINCONF');
   t.is(
     error.message,
     'The "verifyConditions" plugin, if defined, must be a single or an array of plugins definition. A plugin definition is either a string or an object with a path property.'
@@ -73,11 +143,12 @@ test('Throw an error if plugin configuration is missing a path for plugin pipeli
 });
 
 test('Throw an error if an array of plugin configuration is missing a path for plugin pipeline', t => {
-  const error = t.throws(
-    () => getPlugins({verifyConditions: [{path: '@semantic-release/npm'}, {}]}, t.context.logger),
-    Error
+  const error = t.throws(() =>
+    getPlugins({verifyConditions: [{path: '@semantic-release/npm'}, {}]}, {}, t.context.logger)
   );
 
+  t.is(error.name, 'SemanticReleaseError');
+  t.is(error.code, 'EPLUGINCONF');
   t.is(
     error.message,
     'The "verifyConditions" plugin, if defined, must be a single or an array of plugins definition. A plugin definition is either a string or an object with a path property.'