Make it easier to differentiate failures from warnings

We use foodcritic in our deployment pipeline and run all rules, but only fail the build for a subset of them.  I want to make it easier to highlight the failures from the warnings in the build output.

Essentially, I want to be able to do this:

```ruby
opts = {
  :tags => ['any'],
  :fail_tags => ['FC023,FC021']
}

linter = FoodCritic::Linter.new
review = linter.check("cookbooks/", opts)

if review.failed?
  review.failures.each { |f| puts "FAILURE: #{f.rule}: #{f.match[:filename]}:#{f.match[:line]}" }
end
```

This commit is intended to make it easier to determine which warnings are failed vs. not.