optional third button

This may be outside of the scope of what you're going for here, but I added an optional third button to the modal and a corresponding event to trigger.

I've found the project to be really useful in a lot of cases, but have found it wanting for a third button (sometimes more) in each of my use cases.

The class would allow for bootstrap classes such as .btn-warning, etc. I might even suggest adding this kind of option to the OK and Cancel buttons, but to avoid overloading the options array, I'm keeping this PR basic.