Commands should be unregistered upon recompile

CMDHELPER-3063 - Reported by LadyCailin

When register_command is used to override an existing command, upon recompile, the command should be reset to the previous state. Currently, commands registered this way will still remain on the server even if CH is unloaded. Additionally, it would be nice to have a reset_command method, so that the user could trigger this behavior manually as needed.