message: add alias ability for type tags

[MylesBorins]

Adds the ability to append a message using the short form of the type tag, matching how it will be encoded before going over the wire.

e.g.

message.append({
  type: 'integer', 
  value: 123
});

Prior to this change the above would error.

As this functionality never worked before and wasn't expected I am treating this as a new feature rather than a bug fix.