Add minification support for unified js/css files.

If the user puts `app.enable('minify merged');` in an environment config file, then any unified stylesheet or javascript files (concatenated from the previously-existing `merge stylesheets` and `merge javascripts` functionality) will be additionally compressed.

Uses:
- https://github.com/mishoo/UglifyJS2
- https://github.com/GoalSmashers/clean-css