Support custom middlewares

I think it would be useful to have a way to provide custom middlewares for the static webserver. One of the practical use cases is using this plugin in conjunction with https://github.com/intesso/connect-livereload.

I also have implemented two unit-tests based on Mocha. The first one is for base functionality this plugin providing in the version 0.0.1. And the second one for this new feature. The secon test also uses connect-livereload plugin to modify static content.