Add classes to icons to allow for sprites or icon-fonts

Fixes https://github.com/phpbb-extensions/pages/issues/31
The only thing I think needs to be clarified is this really applies to custom styles developers. Because by default icon-pages already adds an icon, which we have already added the ability to over-ride by an admin without changing anything, just adding icons to their styles.. 

To use this would mean either hacking prosilver or other existing styles to add the new CSS, and to over-ride anything the default icon-pages class does. So I don't see this change being useful to the admin, I see it being more useful to style developers. I see admins getting their minds boggled over this :wink: