Merge pull request #1 from nejstastnejsistene/master

Parse @ mentions into links (h/t @nejstastnejsistene)
Targeted high-fives, high-five stealing, Map Channel Message is now Map Channel [Person]
Test script so you can test without Slack at all
Username validation (strip @s) with a newtype
README: features
No high-fiving in private groups
README: shhh
Teach people the targeted syntax