Add support for user-land classname prop

[georgegillams]

At the moment it is not possible to provide a className for the component to apply, as the outer div uses a hard-coded className value. I have updated this so that, with no user-provided className prop it behaves as before, but with a className prop both outer styles will be applied

[travisdmathis]

I will take a look at this after the holidays. Thanks for the submission.