CONCEPTUAL: Support custom config of Highlight language support

Map a Controller to /javascript/highlight.js so we can dynamically
return a highligh.js version based on configured language support.

Configuration of Highlight can be found under Admin > Settings > Highlight.

*DO NOT PULL UPSTREAM*

Would this approach be something to consider for custom language support in highlight configured via Discourse admin settings? With this it's up to the admin to choose specifically which of the n languages to support. 

![screenshot from 2014-10-25 19 45 54](https://cloud.githubusercontent.com/assets/132158/4780579/fc842bde-5c6e-11e4-8d87-d3bcd22b2c54.png)


Missing currently: 
* Some sort of caching of the dynamically generated/joined highlight.js file in the controller?
* Compression of the new highlight.js parts: highlight.js + languages
   * Currently require custom source build of highlight.js
Yeah, I noticed this. It's a bit odd the way it is now. It was included there I assume because it's required by markdown.js: https://github.com/discourse/discourse/blob/master/app/assets/javascripts/discourse/lib/markdown.js#L255

But then dynamically loaded by SyntaxHighlighting later; https://github.com/discourse/discourse/blob/master/app/assets/javascripts/discourse/lib/syntax_highlighting.js#L21

I moved /javascript/highlight.js out as a separate entry in layout/application.html.erb before application.js, so atleast they would load the same file. Second time just from local browser cache.