onremove() should return a promise for actually removing the element

I wish the function onremove() were to return a promise to tell the library to actually remove the element, instead of our having to manually do so in our own implementation.

[okwolf]

@jbucaran how about supporting onremove returning a thunk? This seems like another case of an asynchronous update, except to the DOM instead of state. For example:

// If fadeout takes a callback function
<div onremove={element => remove => fadeout(element, remove)} />

// Or if fadeout returns a Promise
<div onremove={element => remove => fadeout(element).then(remove)} />

So instead of ignoring the return of data.onremove it would look something more like:

function removeElement(parent, element, data) {
  if (data && data.onremove) {
    var remover = data.onremove(element)
    if (typeof remover === "function") {
      remover(function() {
        parent.removeChild(element)
      });
    }
  } else {
    parent.removeChild(element)
  }
}

[Swizz]

We don’t want to support Promise in the core right ? So I approve the idea to make thunk a common practice for async operations.