[IMPR] Enable global args with pwb.py script
[IMPR] Enable global args with pwb.py script

This can be used for tests to set the default site like
 python pwb.py -lang:de bot_tests -v

- Update doc string
- a new method handle_args parses given args and returns the script's
  filename and its options and local pwb arguments which can be passed
  to pywikibot.handle_args
- check whether local pwb options are valid. Otherwise print an error
  and the doc string.

Note: Idf0af67a must be merged first

Bug: T216825
Change-Id: I511604833548f197e8a38dd72c211ebb851db8fb

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