Feature - Ability to use a different unicode symbol for a highlighted star

[skela]

It would be pretty cool if we could use 2 different symbols, for example ★ for when its highlighted and ☆ when its not.

[evianzhow]

@skela @akiroom I've added this feature in the latest commit. But be careful with the demo project since I changed the way to customized the rating view (using dict now). I will updated the demo project if I have spare time. Feel free to check my fork and give me more issue: https://github.com/AustinChou/AXRatingView