Elm Phoenix.Presence Demo

I've spent the last week or so adding support for Phoenix.Presence to elm-phoenix-socket. It's not yet ready for a pull-request, but it's ready to demo. Here you go:

If you wanted to follow along with the development, it's been happening in our Elm topic. Thanks!

Also a big thanks to Fred Yankowski for fixing the bug that I found in Elm's core Dict module during this effort.