Add autocompletion for player arguments to commands

[bhavanki]

Basic command autocompletion was added in 07eedcb. Add completion for command arguments that represent players. This should work by looking through the players visible to the player issuing the command. That list should be generated by running a pseudo-command akin to LOOK, but just for getting that player list. So, this would be a command that is run in order to complete a different command.