Merge branch 'ab/mediawiki-namespace'

The remote-helper for talking to MediaWiki has been updated to
work with mediawiki namespaces.

* ab/mediawiki-namespace:
  remote-mediawiki: show progress while fetching namespaces
  remote-mediawiki: process namespaces in order
  remote-mediawiki: support fetching from (Main) namespace
  remote-mediawiki: skip virtual namespaces
  remote-mediawiki: show known namespace choices on failure
  remote-mediawiki: allow fetching namespaces with spaces
  remote-mediawiki: add namespace support