Add type and optional title to messages

[ajesler]

A message format that provides more information about the context of a message would allow a richer display of the messages, adding images and colourisation.

{
  "body": "Whom would you like to devour?",
  "type": 10
}
// or
{
  "title": "start [--extra-roles ROLE,...] PLAYER ...",
  "body": "Starts a new game with the given players and extra roles. A game requires at least 7 players.",
  "type": 8
}