Support for custom css class for backdrops.

I had a use-case in my project where I needed the backdrops to have different types of visuals, and I thought I would contribute this change to this project.

To use this feature, simple add data-backdrop-class="" to your modal element and it will append the class you specified on to the created backdrop.

I would like to say that this effort is fantastic, for our project it's solved a few things and saving lots of time. Great initiative and keep up the good work.