Adding selection color as an option

I love Selecta! But I'm not a fan of the plain white selection color. So I added a way to pass in a color option. Let me know if you'd like me to make changes, if the code isn't organized to your liking.

![selecta2](https://cloud.githubusercontent.com/assets/648260/5051324/6e78afea-6beb-11e4-8911-308624f8cd69.gif)