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/integration.test.js

@@ -402,6 +402,45 @@ test.serial('Exit with 1 if a plugin is not found', async t => {
   t.regex(stderr, /Cannot find module/);
 });
 
+test.serial('Exit with 1 if a shareable config is not found', async t => {
+  const packageName = 'test-config-not-found';
+  const owner = 'test-repo';
+  // Create a git repository, set the current working directory at the root of the repo
+  t.log('Create git repository');
+  await gitRepo();
+  await writeJson('./package.json', {
+    name: packageName,
+    version: '0.0.0-dev',
+    repository: {url: `git+https://github.com/${owner}/${packageName}`},
+    release: {extends: 'non-existing-path'},
+  });
+
+  const {code, stderr} = await t.throws(execa(cli, [], {env}));
+  t.is(code, 1);
+  t.regex(stderr, /Cannot find module/);
+});
+
+test.serial('Exit with 1 if a shareable config reference a not found plugin', async t => {
+  const packageName = 'test-config-ref-not-found';
+  const owner = 'test-repo';
+  const shareable = {getLastRelease: 'non-existing-path'};
+
+  // Create a git repository, set the current working directory at the root of the repo
+  t.log('Create git repository');
+  await gitRepo();
+  await writeJson('./package.json', {
+    name: packageName,
+    version: '0.0.0-dev',
+    repository: {url: `git+https://github.com/${owner}/${packageName}`},
+    release: {extends: './shareable.json'},
+  });
+  await writeJson('./shareable.json', shareable);
+
+  const {code, stderr} = await t.throws(execa(cli, [], {env}));
+  t.is(code, 1);
+  t.regex(stderr, /Cannot find module/);
+});
+
 test.serial('Create a tag as a recovery solution for "ENOTINHISTORY" error', async t => {
   const packageName = 'test-recovery';
   const owner = 'test-repo';