Configure command used to run tests

The command used to run tests is hardcoded to "ruby". But sometimes you'll want to configure the command to:

1. bundle exec ruby
2. FOO=BAR bundle exec ruby
3. testdrb

Can you please provide a way to configure the command used to run tests, similar to [vim-ruby-test](https://github.com/janx/vim-rubytest)

```
let g:vimux_rubytest_cmd_test = "bundle exec ruby %p"
let g:vimux_rubytest_cmd_testcase = "bundle exec ruby %p -n '/%c/'"
```