Support Elixir's ExUnit

Signed-off-by: Mikhail S. Pobolovets <styx.mp@gmail.com>
Add docs for g:vroom_mix_test_command

Signed-off-by: Mikhail S. Pobolovets <styx.mp@gmail.com>
Merge pull request #52 from styx/master

Support Elixir's ExUnit