Add standard, active classes to slide indicator

[jafaircl]

Adding a standard class to the slide indicators while adding an active class when they're active will let people animate transitions between states. It shouldn't affect anyone's current implementation but will allow indicator animations.

[JoshDSommer]

@jafaircl awesome, thank you for the PR, I'll merge this in and publish this asap.

[JoshDSommer]

Published in version 6.0.1, Thanks again @jafaircl