Add --watch option

[adrienne]

I found this previous issue #11 that was withdrawn by the requester, but i still need it. None of the related tools work for me, since i specifically need to not preserve the directory structure, and cpy is the only node tool i've found that doesn't.

[jhildenbiddle]

I'd also like to see this functionality added.

Specifically, I'm looking for one package to handle watch+copy+rename tasks. Other copy-related packages can do watch+copy but not rename, or copy+rename but not watch.

[sindresorhus]

Sure. Good idea. This should be implemented in cpy first though.