Add an option to reject a message

[michalstocki]

Currently we can only respond to a message, remotely resolving a promise awaiting in the other frame. We'll find it useful if we could also reject a message like this:

talker.onMessage = function(message) {
  message.reject(reason);
};

Then, the reason string will be send to a talker in other window and the awaiting promise will be rejected with the reason.

[Kerrick]

Sounds like a great feature! PRs accepted :)