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

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 = {