Support handling CustomEvents in LiveView

Had another thought just now. What if the phx-custom-events attribute took a value that could be:

  • An event name like MyCustomEvent would cause phoenix to listen for the event and push it up the channel
  • An event name and handle_event name separated by : eg MyCustomEvent:custom_event would cause it to listen for the DOM custom event MyCustomEvent but send it to LV as custom_event
  • A comma separated list of the above

This would mean we only have a single attribute to look for, and the usage is a tad less onerous.