Private fields that would probably be useful?

[ghost]

Hey, great library!
I'm using this to write some small bots, but I'm wondering why the ID and channel fields are hidden here: https://github.com/james-bowman/slack/blob/master/message.go#L28

It would be very handy to have access to channel ID, so that the bot could more easily be programmed to avoid engaging with messages from certain channels, and restrict itself only to channels in which it is relevant?