Ratelimiter doesn't count multiple joins per message

[gempir]

You can send multiple joins per JOIN irc message.

See this function https://github.com/gempir/go-twitch-irc/blob/b173bfca87681979eca791b37aacba5f931428dd/client.go#L610

The ratelimiter doesn't take that into account.

When the client isn't connected yet, go-twitch-irc will "cache" the joins and then write one bigger join message.

• We should count each channel in a JOIN message and add that to the ratelimiter
• limit the joins per message to 20 (that would be easiest, or limit it to the ratelimt) in addition to the current maxMessageLength