feat(plugins): add getLastRelease hook

The code to determine the last-release is now in its own repository: https://github.com/semantic-release/last-release-npm
One can overwrite that behavior by defining a "getLastRelease" plugin. This way
one can decouple semantic-release from npm, e.g. by implementing a git-tag
based version. This is already worked on: https://github.com/semantic-release/last-release-git-tag

Closes #56

src/lib/last-release.js

@@ -1,26 +0,0 @@
-const SemanticReleaseError = require('@semantic-release/error')
-
-const npmlog = require('npmlog')
-const RegClient = require('npm-registry-client')
-
-module.exports = function ({pkg, npm}, cb) {
-  npmlog.level = npm.loglevel || 'error'
-  const client = new RegClient({log: npmlog})
-
-  client.get(`${npm.registry}${pkg.name.replace('/', '%2F')}`, {
-    auth: npm.auth
-  }, (err, data) => {
-    if (err && err.statusCode === 404) return cb(null, {})
-    if (err) return cb(err)
-
-    const version = data['dist-tags'][npm.tag]
-
-    if (!version) return cb(new SemanticReleaseError(`There is no release with the dist-tag "${npm.tag}" yet. Tag a version first.`, 'ENODISTTAG'))
-
-    cb(null, {
-      version,
-      gitHead: data.versions[version].gitHead,
-      tag: npm.tag
-    })
-  })
-}

src/lib/plugins.js

@@ -5,7 +5,8 @@ let exports = module.exports = function (options) {
     analyzeCommits: exports.normalize(options.analyzeCommits, '@semantic-release/commit-analyzer'),
     generateNotes: exports.normalize(options.generateNotes, '@semantic-release/release-notes-generator'),
     verifyConditions: exports.normalize(options.verifyConditions, '@semantic-release/condition-travis'),
-    verifyRelease: exports.normalize(options.verifyRelease, './plugin-noop')
+    verifyRelease: exports.normalize(options.verifyRelease, './plugin-noop'),
+    getLastRelease: exports.normalize(options.getLastRelease, '@semantic-release/last-release-npm')
   }
 }