Custom Interpolators

• Added the option to pass in a custom interpolation function to use when animating between keys.
• Export common easing functions (easeInOutQuart, easeInElastic etc) to use as interpolators

🚨 Breaking changes

A custom input time is now passed in as a property on the options object:

// Animate function API
animate(keys, options: { inputTime?: number, interpolator?: (progress: number) => number });