Missing metadata

[transcental]

Hi! I'm loving this package, it's awesome - thank you so much for making it :)

It's be great if this could support message metadata (https://docs.slack.dev/messaging/message-metadata/)
Currently I'm building messages and manually appending it, but support would be incredible.

[imryche]

Hey! Agree, that would be super handy. I'll add it in for the next release. Thanks for the idea!