Help add new easing functions

We could always use more easing functions.

An easing function is single-variable function that maps values in the range [0, 1] to a numeric value. Generally, we require that f(0) = 0 and f(1) = 1, but otherwise there are no constraints.

If you want to help out feel free to submit a PR, and don't worry about code style. If you need help I can provide a template for your PR.

Tests aren't required for easing functions, but if you want to you can add the new easing function to the graphs example.