assets: requirejs exclude option

* Adds support for exclusion of files in RequireJS builds, to prevent
  files from being included multiple times. (closes #2411)

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Just did some tests and using minified builds saves 116kb over the non-minified builds. 

However I just tried to play with some more aggressive compression for uglifyjs and managed to save 194kb over non-minified builds, so I'll enable more aggressive compression and set back to non-minified files.