Add mappings for public API

This tests that <Plug> maps execute commands This does *not* test that key
mappings (like `<Leader>a`) correctly execute commands, because it's difficult
(and maybe impossible) to send `<Leader>` to Vimrunner.

* Setting `mapleader` doesn't work - `echo mapleader` is always empty.
* Using `vim.feedkeys("\<Leader>a")` doesn't work

https://trello.com/c/LRlRb93n