Core command aliasing

Core commands (those documented by Cadence help) can now be given
aliases - Alternative prompt strings that will be translated to
canonical forms before command processing occurs.

Alias canonicalization will only occur after bans are checked (avoiding
wasted time checking messages from banned users).

Currently configured aliases (by default) are to support a common typo
("Cadence nowplaying"), and to support one alternate usage of play (to
perform requests) that was brought to my confusion during some confusion
regarding the helptext that should be written for play.