Help add new easing functions

[mikebolt]

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.

[trusktr]

Maybe something neat would be an easing function primitives for creating easing functions, like what popmotion has.

[dalisoft]

@mikebolt What about https://www.npmjs.com/package/owneasing? Maybe helps user create easing by curving which can be created by almost any SVG editor

[dalisoft]

Or this also may be useful https://www.npmjs.com/package/es6tween-physics

[trusktr]

Nice! Let's evaluate how we may incorporate those.

[trusktr]

For reference, here's the functions in popmotion for creating new curves: https://popmotion.io/api/easing.