Add support property classnames as object.

[Lol9tor]

As it works with CSSTransition, we should make this works here. Object will look like that:

{
  classNames: {
    enter: 'classname1',
    enterActive: 'className2',
    exit: 'className3',
    exitActive: 'className4'
  }
}

We need it for using this transition with css modules

[nwalters512]

I've started working for the summer so I'll have somewhat limited time to work on this. Feel free to submit a PR in the meantime!