Support connecting to an IRC network with multiple hostnames

[StalkR]

NewBot API supports a single hostname (https://github.com/StalkR/goircbot/blob/master/bot/bot.go#L35), we'll need to change that, possibly introducing a slice of host, port and SSL (maybe in a string if we use host:+port format where + is optional for SSL).
And to be able to reconnect, we'll need to keep that slice somewhere in BotImpl https://github.com/StalkR/goircbot/blob/master/bot/bot.go#L99

goirc client.Config struct supports a single hostname (https://github.com/fluffle/goirc/blob/master/client/connection.go#L65). Possibly we can keep that.

We'll need to think carefully about this as it's a breaking API change, so I'd prefer to get it right first time.

[StalkR]

Now that we have #21 functional options thanks to @quite should be easy to add.