[Request] Method to reply/message to specific Slack user

[jahirfiquitiva]

Hi

Thanks for creating this awesome lib. I'm going to create a couple issues for feature requests I want to make for this project

I hope they get implemented soon

Thanks in advance

[mizoguche]

Thank you for your creating issues.

I'll implement those requests!

[jahirfiquitiva]

@mizoguche Thank you so much :D