I've added allow_attributes functionality.

I found this function missing while using wysihtml5 with bootstrap (I found it within x-editable package so I first used it with bootstrap). I've added option to ignore declared attributes and it's support in parserRules. Forgive me my lack of github etiquette as I'm a new user here.

Add in parserRules "allow_attributes": {"data-toggle":1}to your tag declaration.

Signed-off-by: falxcerebri <lukasz@groszkowski.pl>