Gracefully handle session termination and reconnect

[Privat33r-dev]

Original issue: play-with-docker/play-with-docker.github.io#246

Reconnect

Screenshot

When server starts `reconnect` and then `connect`s again, it resends full input starting from the start of the session. The app is currently just ignoring "reconnect/connect" and sends the output from the start as is (see screenshot above).

The graceful handling of this case would be cleaning up previous input before setting received one (user's current input in the readline can be kept).

I guess that the code would look like somewhat like this:

    this.socket.on('reconnect', function(name ,data) {
      var instance = self.instances[name];
      if (instance && instance.terms) {
        instance.terms.forEach(function(term) {term.clear()});
      } else {
        self.instanceBuffer[name] = '';
      }
    });

I am not exactly sure though on this suggestion and would advise to refer to the protocol's documentation first.

"Session end"

{"name":"instance delete","args":["INSTANCE_ID"]}
{"name":"session end","args":null}

On receiving this input the client is expected to visually indicate to the user that session finished instead of keeping unresponsive console UI. Ideally, session restart should be offered.