Allow users to supply own update method

[WickyNilliams]

It would be nice if you could supply own update method. In my use case I am using the accelerometer to govern progress (e.g. shaking phone increases velocity, and thus progress along line. progress is not tied to time or frames, but to user's shake movement).

By allowing users to supply their own update function, the lib gets more flexible and there's an opportunity to reduce code duplication between animate_frame and animate_duration.

You can provide some higher-order functions to replace the internal animate_frame/duration functions:

metronome.timeBasedUpdate = function(duration) {
  var start = Date.now();

  return function() {
    return Math.min(1, (Date.now() - start) / duration);
  };
}

metronome.frameBasedUpdate = function(duration) {
  var frame = 0;

  return function() {
    return Math.min(1, (frame++ / duration));
  };
}

then you could use metronome like so:

metronome({
  update : metronome.frameBasedUpdate(200)
  render : function() {} // whatever
})

metronome({
  update : metronome.timeBasedUpdate(2000)
  render : function() {} // whatever 
})