Clean up mime.py a tad. Stop implementing our own splitext().
Allow specifying a regex for bugfinder plugin
Add plugin_buglink_url as an alternative to plugin_buglink_bugzilla for buglink plugin
Fix style in buglink htmlifier.

Declare all globals explicitly. PEP 8 stuff.
Fix the mime.py routine I broke.
Factor out a little repetition.

One symbol binding, one call to re.compile ftw.
Allow custom regexes for finding bug mentions. Close #158.

Also, rename a config option to not sound Bugzilla-specific.
Add a favicon. Fixes bug 940711.

It's a PNG we had lying around, so it won't work in Safari<4 or IE<11, and it's 16x16, so it won't look its best on Retinas, but it gets pinned tabs recognizable, so it's a step forward!