Maintain custom select classes

This fixes #611 by @chrise86.

Any classes on the `<select>` element will be appended to the generated `<div class="custom dropdown">` element.

This may or may not be wanted, but I thought I'd create a patch for it anyways since I certainly find it useful. I've also updated the docs with a line explaining how this works, not sure if it's in the right place or not.