Add irc_output to heka

+1 to not putting everything in one giant file, but all of the other plugins have the plugin struct, the config struct, and the ConfigStruct() method in the same file as the rest of the plugin implementation, I'd like to maintain that pattern. Also, it's nice to have constructors (like your NewIrcConn function, below) immediately following the struct or interface definition of what they return (your IrcConnection interface, above).