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

package.json

@@ -24,12 +24,12 @@
     "@semantic-release/commit-analyzer": "^2.0.0",
     "@semantic-release/condition-travis": "^4.0.0",
     "@semantic-release/error": "^1.0.0",
+    "@semantic-release/last-release-npm": "^1.1.1",
     "@semantic-release/release-notes-generator": "^2.0.0",
     "git-head": "^1.2.1",
     "github": "^0.2.4",
     "lodash": "^3.9.3",
     "nopt": "^3.0.3",
-    "npm-registry-client": "^6.4.0",
     "npmconf": "^2.1.2",
     "npmlog": "^1.2.1",
     "parse-github-repo-url": "^1.0.0",

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')
   }
 }
 

src/pre.js

@@ -2,14 +2,14 @@ const _ = require('lodash')
 const auto = require('run-auto')
 const semver = require('semver')
 
-const getLastRelease = require('./lib/last-release')
 const getCommits = require('./lib/commits')
 const getType = require('./lib/type')
 
 module.exports = function (config, cb) {
-  const {plugins} = config
+  const { plugins } = config
+
   auto({
-    lastRelease: getLastRelease.bind(null, config),
+    lastRelease: plugins.getLastRelease.bind(null, config),
     commits: ['lastRelease', (cb, results) => {
       getCommits(_.assign({
         lastRelease: results.lastRelease

test/specs/pre.js

@@ -6,9 +6,16 @@ const pre = proxyquire('../../dist/pre', {
   'child_process': require('../mocks/child-process')
 })
 
+const versions = {
+  available: '1.0.0'
+}
+
 const plugins = {
   verifyRelease: (release, cb) => cb(null, release),
-  analyzeCommits: (commits, cb) => cb(null, 'major')
+  analyzeCommits: (commits, cb) => cb(null, 'major'),
+  getLastRelease: ({ pkg }, cb) => {
+    cb(null, { version: versions[pkg.name] || null, gitHead: 'HEAD' })
+  }
 }
 
 const npm = {