Configurable toolbar classes

This is https://github.com/xing/wysihtml5/pull/461 which I wrote last year some
time, rebased on to your fork.

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.