title: PostMessage with multiple functions or custom callbacks
body: <p>So far I've only seen tutorials for postmessage where one window sends a single kind of message, and the other window interprets the message in only a single way.</p>
  
  <p>What if I want to have many different kinds of interactions between windows, can postmessage handle that?</p>
  
  <p>Is that going against the grain of what postmessage is supposed to do?</p>
  
  <p>For example, what if I wanted to be able to send custom callbacks back and forth, etc?</p>