feat(plugins): normalize plugins and have unified function signature

src/lib/error.js

@@ -1,6 +1,7 @@
 module.exports = class SemanticReleaseError extends Error {
   constructor (message, code) {
-    super(message)
+    super()
+    this.message = message
     this.code = code
   }
 }

src/lib/plugin-noop.js

@@ -0,0 +1,4 @@
+/* istanbul ignore next */
+module.exports = function (options, release, cb) {
+  cb(null)
+}

src/lib/plugins.js

@@ -0,0 +1,17 @@
+let exports = module.exports = function (source) {
+  return {
+    analyze: exports.normalize(source.analyze, '@semantic-release/commit-analyzer'),
+    generateNotes: exports.normalize(source.analyze, '@semantic-release/release-notes-generator'),
+    verify: exports.normalize(source.verify, './plugin-noop')
+  }
+}
+
+exports.normalize = function (plugin, fallback) {
+  if (typeof plugin === 'string') return require(plugin).bind(null, {})
+
+  if (plugin && (typeof plugin.path === 'string')) {
+    return require(plugin.path).bind(null, plugin)
+  }
+
+  return require(fallback).bind(null, plugin)
+}

src/lib/type.js

@@ -1,16 +1,18 @@
 const SemanticReleaseError = require('./error')
 
 module.exports = function (plugins, commits, lastRelease, cb) {
-  const type = plugins.analyze(commits)
+  plugins.analyze(commits, (err, type) => {
+    if (err) return cb(err)
 
-  if (!type) {
-    return cb(new SemanticReleaseError(
-      'There are no relevant changes, so no new version is released',
-      'ENOCHANGE'
-    ))
-  }
+    if (!type) {
+      return cb(new SemanticReleaseError(
+        'There are no relevant changes, so no new version is released',
+        'ENOCHANGE'
+      ))
+    }
 
-  if (!lastRelease.version) return cb(null, 'initial')
+    if (!lastRelease.version) return cb(null, 'initial')
 
-  cb(null, type)
+    cb(null, type)
+  })
 }