Add onUpdate callback

[jamesheazlewood]

Because the container calculation is wrapped in a setTimeout callback, it is difficult to trigger anything after this is called. Adding an onUpdate callback inside there allows easily hooking into this.

Updated the docs to include this also.

[skmbr]

I was just about to do something like this myself... Would be great if this got merged in!

[sandstrom]

@skmbr Can you checkout this branch and see if it works for your use-case?