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/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