Add a watch option to the cli utility

[peacememories]

Hi, and first of all thanks for making such a useful preprocessor =)

I've recently added support for additional commands to the elm-live server. The goal was to use elm-css within an auto-building environment.

While this works somewhat (the syntax is not great yet), but I feel a better approach would be to start the live-server and watching builders in parallel. For this, elm-css would of course need to support watching files :)

[rtfeldman]

Closing in favor of #327