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/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')
   }
 }