custom arrows

[bgrayburn]

I'd like to be able to use different symbols for the arrows as well as controlling their size (which I believe I can currently do from CSS but would love to be able to control this with a react property). Let me know if you're open to these changes.

[davidsonsns]

It would be interesting to insert an element/component in place of the arrow icon.

If you agree, I can help with that.