Add a config option to attach local classes

[keithamoss]

To pin the Site Switcher to the top of the page on Jabberwocky I need to add navbar-fixed-top is-fixed is-visible to the <nav> element.

Can we add an option to GH.opts that supports adding additional classes?

Alternatively: If GH-SS can fire an event when it's loaded that would work too.

Something to let me replace my horrible workaround hack.