Compile external assets into their own files.

[gdotdesign]

This PR resolves issues with packages having external files:

• external javascripts are now compiled into a single file external-javascripts.js (in development and build)
• external stylesheets are now compiled into a single file external-stylesheets.css (in development and build)
• external files are no longer being restricted to the public directory

With the change of #155 the following issues cropped up:

• packages could not serve their external stylesheets in development because we relied on them being in the public directory
• having restrict external files into the public directory made it impossible to reference files outside of the project. To give this more context: I'm working on mint-ui and mint-ui-showcase, I needed to access a JavaScript file which is mint-ui from mint-ui-showcase as (../mint-ui/assets/scroll_into_view.js) and this was not possible