Add JS asset path for custom actions (Micro PR)

I as a plugin author want to add javascripts to my distributable custom action.

This simply requires the app/assets/javascripts/rails_admin/plugins directory, so a custom action author can place scripts there and doesn't have to override a user's ui.coffee

I would also love this feature for stylesheets but since sass doesn't have a require_tree, that could be a little more difficult.