Add support for guard halt_on_fail

When using guard groups, sometimes it's nice to stop other guards from running during the red-green-refactor cycle when the tests are not passing.

This pull request adds the ability to use guard-minitest in such manner, as to be able to configure it like:

```ruby
group :red_green, :halt_on_fail => true do
  # guard minitest config

  # other guards
end