Add `sender` and `printToSender(<message>)` to `TimecodeDsl`

[RyanHecht]

Add sender and printToSender(<message>) to TimecodeDsl

A common pattern to write a show that debugs something is to do as follows:

show { playerName -> 

def resultOfTest = testSomething();

player(playerName).sendMessage(testSomething);

}

Calling this show is done via /show start debugShow --args "RyanHecht_"

It would be nice if sender was automatically resolved to the CommandSender that started the show. Likewise, printToSender would be a nice convenience method to print strings to the show's starter.