Add a disabled condition which prevents 'open' from doing anything

Status: **Ready for merge**
Reviewers: @kpeatt @tedtate 

## Changes
- Add 'DISABLED': 'bellows--is-disabled' to cssClasses object
- Add new OR condition to 'open' method which returns if $item.hasClass(cssClasses.DISABLED) returns true

## Testing
- Add bellows--is-disabled class to .bellows__item and click it -- and it shouldn't open