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
Yes, the dynamically generated file should be stored on disk in the tmp directory. no reason to regenerate it over and over. 

if you name it "highlight_CONTENTHASH" and ship that hash to the client you solve the caching problem. 

Regarding the location, we are following very similar techniques for avatars so I prefer to keep the code close.