Add engine class methods for registering assets

Created by: gracewashere

See https://github.com/thoughtbot/administrate/issues/566

Problem:

When developing a plugin for Administrate, there is not a good way to define custom javascripts or stylesheets to be included on admin pages.

Existing solutions either use content_for at the field level, or require the end user to override Administrate's javascript partial.

Solution:

Add methods to Administrate::Engine to keep track of the assets that need to be loaded on each admin page.

Plugin developers can now use these helpers in their engine definition:

Administrate::Engine.add_javascript "my_plugin/script"
Administrate::Engine.add_stylesheet "my_plugin/styles"