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 `ENABLE_DOCS=true ember serve` you can access the documentation going to the `/docs` url in the browser. It watch for changes so its always up to date.

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 enable it with an environment variable.

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.