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.

lib/plugins/index.js

@@ -1,11 +1,12 @@
 const {isArray, isObject, omit} = require('lodash');
-const DEFINITIONS = require('./definitions');
+const SemanticReleaseError = require('@semantic-release/error');
+const PLUGINS_DEFINITION = require('./definitions');
 const pipeline = require('./pipeline');
 const normalize = require('./normalize');
 
-module.exports = (options, logger) =>
-  Object.keys(DEFINITIONS).reduce((plugins, pluginType) => {
-    const {config, output, default: def} = DEFINITIONS[pluginType];
+module.exports = (options, pluginsPath, logger) =>
+  Object.keys(PLUGINS_DEFINITION).reduce((plugins, pluginType) => {
+    const {config, output, default: def} = PLUGINS_DEFINITION[pluginType];
     let pluginConfs;
     if (options[pluginType]) {
       // If an object is passed and the path is missing, set the default one for single plugins
@@ -13,18 +14,18 @@ module.exports = (options, logger) =>
         options[pluginType].path = def;
       }
       if (config && !config.validator(options[pluginType])) {
-        throw new Error(config.message);
+        throw new SemanticReleaseError(config.message, 'EPLUGINCONF');
       }
       pluginConfs = options[pluginType];
     } else {
       pluginConfs = def;
     }
 
-    const globalOpts = omit(options, Object.keys(DEFINITIONS));
+    const globalOpts = omit(options, Object.keys(PLUGINS_DEFINITION));
 
     plugins[pluginType] = isArray(pluginConfs)
-      ? pipeline(pluginConfs.map(conf => normalize(pluginType, globalOpts, conf, logger, output)))
-      : normalize(pluginType, globalOpts, pluginConfs, logger, output);
+      ? pipeline(pluginConfs.map(conf => normalize(pluginType, pluginsPath, globalOpts, conf, logger, output)))
+      : normalize(pluginType, pluginsPath, globalOpts, pluginConfs, logger, output);
 
     return plugins;
   }, {});