Add support for displaying an icon before or after the tab text

I've received a few inquiries for this and I think it will fit well as a new enhancement in this plugin. Supporting an icon font like Font Awesome could be the way to go.

• add settings option: icon_name
• add settings option: icon_position (store none/before/after)
• enqueue Font Awesome if icon_position != 'none' and icon_name != ''
• write icon in the tab at proper position