Add plugin bindings to allow for "direct connections"

[geoffxy]

The agent still supports the ConnectTo message, but the vim/neovim plugins no longer send it because we use the hole punching messages.

It may be useful to have a plugin binding that sends this message so we can establish a "direct connection" (e.g. for debugging or if your editing group members are all routable under a private network). This will also require the plugin to display the port number that it's listening on.

[rageandqq]

Sublime also no longer sends / accepts the ConnectTo message as of #105 .