Support recompilation with file watching

[iand675]

It would be really great to be able to tell shake to keep running, and rebuild any time file changes affect a needed output. I'd like to set up a website asset compilation pipeline that remakes everything upon saving a file, but to do so with shake at the moment I more or less have to duplicate the recompilation rules with another tool that calls shake.

[ndmitchell]

I think using the Live files feature you should be able to do something: http://neilmitchell.blogspot.co.uk/2015/04/cleaning-stale-files-with-shake.html. But I want it to be "easy", there's some request for that in #161.

[ndmitchell]

Also requested by @rvion in #441.

[HaoZeke]

Is this still under consideration? For smaller projects, having to write inotify rules and then call shake is a deal breaker..

[chrismwendt]

I'm implementing file watching in #839