Configurable toolbar classes

This patch allows the classes set on toolbar buttons to be set in the wysihtml5 configuration, as `classNameCommandDisabled`, `classNameCommandsDisabled`, `classNameCommandActive`, `classNameActionActive`. I already have `disabled` and `active` classes provided by Twitter Bootstrap and don't want to trawl through the source files and apply all relevant styles to wysihtml5-specific classes too -- this is a very nice short cut.