Set nav "active" class according to url match

It's repetitve and ugly and not even particularly robust, but it's a
start and there's only so much you can do at the template level (we
could start encoding stuff in _config.yml but that just makes
maintenance harder than it needs to be).
Merge pull request #6 from exonian/feature/show-active-nav-pill

Moar dynamicness for nav tabz