Add Gulp Build [Builds on #728]

I took the excellent work from https://github.com/prose/prose/pull/728 and changed it up a bit.

I separated it so that the tasks are in their own separated files and are cleaner / easier to run.

I also want to add a simple gulp develop command or something to that effect to just start development.

@PabloVallejo and others -- would love some feedback.  Thanks!