Checkbox multi

Multi Checkbox field type - based on patch from @stompweb

Like checkbox, except allow more than one. Pass an array of options - like you do with radio. Saved as an array, with checked fields set. If none checked, no data is saved.

* The other frameworks have this.
* Need separate field to keep current behaviour of checkbox (set/unset) 

Question - do we need this over just using multiple single checkbox fields? Better UI?