New Option: add additional css class(es) to close link

I know your intentions are to keep this as absolutely simple as possible, but I'd love the ability to add a custom class to the close button.

This way custom CSS can be written to override the default look for the close button, including the ability to utilize icon fonts. This is especially handy if they are already being used in the project at hand (which, for me, nowadays is almost always.)

This pull represents the simplest implementation of this feature that I could think of, leaving the styles completely up to the user.

An example has been added to the demo, and the README has been updated as well.