Serve documentation live from /docs endpont (optional)

I've been using this in ember cpm for edit documentation quickly and see the result instantly. I liked it, maybe you too.

When you run `ember serve --docs` you can access the documentation going to `/docs`. It watch for changes.

This is disabled by default. Since generating the documentation is an expensive task and most likely you don't want to add ~4s to each rebuild, you need to add the `--docs` flag to `ember serve` to enable it.

Usually in ember-cli apps just installing the addon `ember-cli-yuidoc` adds this behavior out of the box, but in this case it had to be invoked manually in the brocfile.