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.
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.