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' }

**Choosing icon library**
The default icon library is glyphicons, as that is the default with Bootstrap. Too change to Font Awesome, for example:
$('textarea').markdown({iconLibrary: 'fa'})

Of course, still backwards compatible with the old syntax.