Add `ease` function

[tmjoen]

Use gsap-style naming?

[tmjoen]

https://github.com/thoughtbot/bourbon/blob/master/core/bourbon/library/_timing-functions.scss

[tmjoen]

ease: "cubic-bezier(1, 0, 0.3, 1)"
easeOut: "cubic-bezier(0.165, 0.84, 0.44, 1)"
easeIn: "cubic-bezier(0.755, 0.05, 0.855, 0.06)"
default: "cubic-bezier(0.25, 0.1, 0.25, 1)"
easeInOut2: "cubic-bezier(0.455, 0.030, 0.515, 0.955)"
easePower2: "cubic-bezier(0.77, 0, 0.175, 1)"
easePower3: "cubic-bezier(0.645, 0.045, 0.355, 1)"
easePower4: "cubic-bezier(0.770, 0.000, 0.175, 1)"
easeSine: "cubic-bezier(0.445, 0.05, 0.55, 0.95)"
easeInPower2: "cubic-bezier(0.550, 0.085, 0.680, 0.530)"
easeInPower3: "cubic-bezier(0.550, 0.055, 0.675, 0.190)"
easeInPower4: "cubic-bezier(0.895, 0.030, 0.685, 0.220)"
easeOutPower2: "cubic-bezier(0.25, 0.46, 0.45, 0.94)"
easeOutPower3: "cubic-bezier(0.215, 0.610, 0.355, 1)"