Provide hook for determining command access.

[chevcast]

Shotgun should provide a hook before invoking command modules so that the implementing application can use the hook to check permissions for command modules.

For example: In one app I'm building I created a LOGIN command module that sets a cookie and logs the user in. Once the user is logged in I'd like to just remove the login command from that user so that it cannot be accessed and does not show in the help menu.

[chevcast]

Done.