Vastly improve the README

https://trello.com/c/GeYamqK0
Promote vim-tmux-runner
Correctly explain VtrSendCommand
It's obvious we're talking about features
Put our best foot forward
PR feedback
Add mappings for public API

https://trello.com/c/LRlRb93n
WIP: mappings
Add mappings for public API

https://trello.com/c/LRlRb93n
Use maparg
Only create mappings if user has not created them

https://trello.com/c/L41OwR4B
Extract MapIfUnmapped