Have RegexCommand.parse return more information

[MattPrit]

Required for #90
The more complex MultiCommandInterpreter requires more information to be returned form parse. Specifically, it requires information about where within the message a match is found; these indices need to be with respect to the original message, not (if applicable) the message after it has been decoded. This could be achieved by encoding the regex pattern rather than decoding the message when format is not None.
Since there will be changes to the return signature of Command.parse, the handle method of CommandInterpreter will need to be updated.