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>
I am in favour of always adding the non-minified version of the scripts and only minifying them for production. Makes life much easier when debugging certain problems.