[module] Added Decorator 'require-privmsg' to allow commands to be specified as only available via PM (Private Message)

Signed-off-by: Eldon McGuinness <eldon.mcguinness@gmail.com>
Merge pull request #390 from EldonMcGuinness/add-require_privmsg-decorator

[bot][module] Added Decorator 'require-privmsg' to allow commands to be specified as only available via PM (Private Message)