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>
Without having look too much into it, it looks like that our minification process generated larger files than the minified builds. That was the main reason for the change, but naturally it would be better to figure out why our builds are larger.