Add pygments parser option

global now builds a pygments parser by default. Make this work with a --with-pygments option to the recipe.
Ansible is a python package, global is not. Global (gtags in this case) is running a python script pygments_parser.py which imports the pygments module. I modified the recipe to be like ansible (and thus pygments is installed in global's cellar), but the pygments_parser.py cannot find the installed python package when gtags runs it. Not sure how to make this work.