Add `feature_root` config for release dir

Allow you to serve all your static assets from a sub directory of the release
instead of `./`. That way, you can specify if you’d like to only make files
from e.g. `dist/` available instead of the complete feature directory.

Bonus: Deploy tools like Capistrano can now also be used to publish your assets
by specifying its `current` symlink as the feature’s root.