Add new special format specifiers.

Same as SourceMod, this adds 2 special format specifiers:
* ```%n``` : expands to a string containing the player's name.
* ```%N``` : expands to ```1<2><3><4>``` where ```1``` is the player's name, ```2``` is the player's userid, ```3``` is the player's SteamID, and ```4``` the player's team name.

I would have been nice to use ```%L``` for log format, but unfortunately it's already taken.
If you have better suggestion for a letter, @TheDS, fee free to share it.