Add :tablet format by default

[petRUShka]

There is a common option: separate tablet view from mobile view. It isn't convenient to filter tablet by user code. It is better to detect tablet devices inside mobylette like it was done in mobile_fu. You can get regexp for tablet from this file: https://github.com/benlangfeld/mobile-fu/blob/master/lib/mobile-fu/tablet.rb#L5