add utilize to "autodetect" best watcher.

[stefanpenner]

Maybe something like: https://github.com/chrmod/watch-detector/blob/master/lib/index.js

api could be:

sane.auto(/*... existing API...*/)
// or

sane(root, { auto: true });

depends on: #123

Question:

• api choice?
• should we inline watch-detector into this project entirely? Since it really just exists for sane