mwgrep: Add namespace prefix in output

This saves many lives.

Before:
 $ mwgrep ipv6wwwtest
 liwiktionary        Common.js
 $ mwgrep pgcount --user
 trwiki              Krutata/common.js
 mznwiki             پسر آمل/movaghat.js
 elwiktionary        Dada/monobook.js
 arwiki              ميموني/cologneblue.js
 hrwiktionary        Ivan Štambuk/monobook.js

After:
 $ ./mwgrep ipv6wwwtest
 liwiktionary        MediaWiki:Common.js
 $ ./mwgrep pgcount --user
 trwiki              User:Krutata/common.js
 mznwiki             User:پسر آمل/movaghat.js
 elwiktionary        User:Dada/monobook.js
 arwiki              User:ميموني/cologneblue.js
 hrwiktionary        User:Ivan Štambuk/monobook.js

Quicker to copy/paste and actually use in some fashion.
Next step, print the actual domain (or full url even) instead
of just the dbname.

Change-Id: I9b3f58390ca4ba6d2b4dcc176fa11515151441c6