template: @template_args decorator addition

* Adds new decorator to allow overwritting template context functions by endpoint.  (closes #2009)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
We should add a note that string version of ``template_args`` should only be called from an extensions setup_app. Using it in a blueprints before_app_first_request will not work.