mwgrep: add '--user' option for searching NS_USER

When mwgrep is invoked with the '--user' command-line argument, search NS_USER
rather than NS_MEDIAWIKI.

I chose to make this a flag rather than allow the exact namespace to be
specified via an argument because:

* it minimizes the chances of an accidental NS_MAIN search
* searching NS_MEDIAWIKI is a sensible default
* though not prohibitively expensive, searching NS_USER is onerous enough
  that it should only be performed if explicitly required

Change-Id: Idaad02b78bc43c191b1fa8a877a08acd97bb52a5