add utilize to "autodetect" best watcher.

Maybe something like: https://github.com/chrmod/watch-detector/blob/master/lib/index.js

api could be:

```js
sane.auto(/*... existing API...*/)
// or

sane(root, { auto: true });
```

depends on: https://github.com/amasad/sane/issues/123

Question:

* api choice?
* should we inline watch-detector into this project entirely? Since it really just exists for `sane`