Pagegenerators.py: make grep a recognised key of GeneratorFactory.handleArg()

Replace.py does not understand that "grep" is a pagegenerators key
and uses -grep:xxx arg as replacement.
[see  "# Read commandline parameters" section in main()]

Change-Id: Iabe5caae61f5fab3151c97ed1900678ece58e655
Merge "Pagegenerators.py: make grep a recognised key of GeneratorFactory.handleArg()"