Option for other icon libraries

Glyphicons are great and all, but I know multiple websites use other icon libraries (most commonly Font Awesome). Simple implementation:

**Old Icon Syntax**
icon: 'glyphicon glyphicon-bold'

**New Icon Syntax**
icon: { glyph: 'glyphicon glyphicon-bold', fa: 'fa fa-bold' }

Of course, still backwards compatible with the old syntax.