First draft: validate watch files

[gaku-sei]

Closes #36

Revalidates project on file change. Notice that since notify (the library we use for watching/executing) doesn't support globs the user must provide the path twice (also, they could be different and in the future I think it would make sense to accept a list of -w or to have it in a config file (#35)).

Seems to work well and the dev experience even on large projects (~5000 files of ~650 lines each) is smooth.

Some things to improve:

• Code (it's a bit rough and basically copied from the generate command)
• Ownership is not good (we send arc here and there, clone them when not needed, watch path is not computed when it should etc...)
• Not exit on file rename (except the file is the css file), just keep going
• Better error handling