feat(type): initial functionality

src/lib/error.js

@@ -0,0 +1,6 @@
+module.exports = class SemanticReleaseError extends Error {
+  constructor (message, code) {
+    super(message)
+    this.code = code
+  }
+}

src/lib/type.js

@@ -0,0 +1,19 @@
+const SemanticReleaseError = require('./error')
+
+module.exports = function (plugins, results, cb) {
+  const commits = results.commits
+  const lastRelease = results.lastRelease
+
+  const type = plugins.analyze(commits)
+
+  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')
+
+  cb(null, type)
+}