improvements to unlink.py

- UnlinkBot extends pywikibot.Bot
  with built-in 'always' option
- BREAKING: require the page name to be wrapped in quotes
- use core-like page.save
- remove obsolete script name from showHelp
- make localized strings depend on the actual working site

Change-Id: I24757c086d1eda88d71148a547aebaa8aeca56ec
Merge "improvements to unlink.py"