Loop mode support "while" argument

[sasha240100]

This issue is an addition to issue #9

Would be nice if .loop() would support aka "while" function instead of specific maxNumber.

Concept:

let isLoaded = false;

fetch('http://example.com/api').then(() => {
  // do some stuff
  isLoaded = true;
})

const tween = new Between(0, 360)
  .time(1500) // 1500ms
  .loop('repeat', () => !isLoaded); // repeat if not loaded

tween.on('update', v => {
  spinnerElement.style.transform = `rotate(${v}deg)`;
});