Start --watch integration

[tmcw]

Questions:

• Is chokidar a bad dependency to have? Being a binary install, does it matter? Right now I'm depending on mdast also requiring chokidar such that "we'll have it anyway"

TODO

• externalize dependency finding to give chokidar an accurate list of files to watch
• solve debounce problem
• implement http mode for html output (including the vinyl virtual problem ( [question] vinyl virtual adapter gulpjs/vinyl#69 )
• determine how to handle failure
  • upstream Desired background? Tjatse/ansi-html#2
• change CLI to accept [build|serve|lint]
• deprecate dev-documentation

Fixes #215

[tmcw]

Okay, this is ready for a review, cc @jfirebaugh or @mourner: this pulls the functionality of dev-documentation into core: new -w and -s flags respectively watch source files for updates and serve them over http locally.

[jfirebaugh]

Can we jump ahead a few CLI revisions and do the usual command pattern that everyone seems to settle on:

documentation build
documentation serve [--watch]
documentation lint

[tmcw]

Would omitting a command show help, or default to build?

[tmcw]

Also, so far I am permitting for the possibility of --watch working for both building & serving - so, for instance, you could auto-rebuild a Markdown doc too.

[jfirebaugh]

Show help is what jekyll does, sounds good to me.

👍 on documentation build --watch too.

[tmcw]

Back to you for review @jfirebaugh, branch now passes tests and needs a line or two of coverage improvement.

[jfirebaugh]

I don't have time to do a detailed review here. If you're happy, 🚢.