* Make serverless-webpack more extensible
Use PluginManager to make serverless-webpack more extensible
Following the advice from serverless blog on Advanced Plugin Development. https://serverless.com/blog/advanced-plugin-development-extending-the-core-lifecycle/
* use sublifecycle with webpack command
* Use new lifecycles and adapt unit tests
* Fixed unit test after merge
* Use new events everywhere
* Added section to README