Turn wikimedia_commons values into links

This turns the value of the `wikimedia_commons` tag into a link to the respective page on `commons.wikimedia.org` (if it is a `File:` or `Category:` item) by enhancing the browse helper. See [1] for the tag description.

(Btw.: I have no hard feelings if this is rejected - I recently noticed this tag (which is not that popular yet, but the wiki page [1] is also fairly new), found it useful (since it seems to solve some of the issues of the `image` tag if images are available on Wikimedia Commons), and wanted to see how easy/hard it is to implement such a link.)

[1] http://wiki.openstreetmap.org/wiki/Key:wikimedia_commons