Add support for width formatting in nick

Adds support for specifying a minimum width for nick display. For example with
a nick format of "<%-9n>" the nick "bd808" will be rendered as "<    bd808>".

Code taken from
https://github.com/psychs/limechat/blob/master/Classes/IRC/IRCClient.m at hash
4a1e187dc81cabd5bba35a0a26ea7ab3c963326e