Allow tests to be run from nvim

[camagu]

The current vim-rspec + dispatch impleemntation is broken, plus it'll be cool to
integrate with other test commands and languages.

https://github.com/kassio/neoterm looks interesting but it doesn;t seem to
integrate with the quickfix window.

[camagu]

https://github.com/benekastah/neomake looks really cool.

[camagu]

https://github.com/janko-m/vim-test is the correct one, not neomake