Snippets/Load JS and CSS by URL

Description
Adds support for loading a script or stylesheet from a wiki page in the MediaWiki namespace, based on a URL parameter. This makes showing demonstrations easier as users don't have to enable a gadget or put something in their user.

Code
This version adds two new url parameters withJS and withCSS. You can append them on to the end of the url: ?withCSS=MediaWiki:Foo.css&withJS=MediaWiki:Foo.js

Load multiple files
This version adds a single url parameter, use but allows multiple files to be separated by a pipe (|). You can then append on to the end of a url: ?use=MediaWiki:Foo.js|MediaWiki:Foo.css|MediaWiki:Bar.js