Cubic Bezier array easing support

This release includes a new feature from @hoomanaskari (#171). Now you can specify an array of numbers to represent a cubic Bezier curve! Check out the updated API documentation for this feature:

• https://jeremyckahn.github.io/shifty/doc/shifty.html#.tweenConfig
• https://jeremyckahn.github.io/shifty/doc/tutorial-easing-function-in-depth.html