category.py: Add option -append to listify
category.py: Add option -append to listify

Added option -append to append list to already existing page content
when listifying.

Tested: https://www.mediawiki.org/wiki/User:Alangi_Derick/Sandbox/listify3

Bug: T197841
Change-Id: I15860189a2044caecec4750247cee4008a9bb0a9

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core