Custom snapshot callbacks

[kettanaito]

What

Need to consider custom callbacks (or multiple callbacks) for a snapshot.

Why

In case #3 or similar are implemented, they would need a respective callback functions.

How

new Observer({
  snapshots: [
    appear() {},
    disappear() {}
  ]
});

This may as well be combined with #18 so a resolver can return an object of which callback function it should call and whether it should call it under the current snapshot condition.

new Observer({
  snapshots: [
    resolver: (...) {
      return { should: Boolean, callback: 'myCallback' };
    },
    myCallback() {}
  ]
});