Allow passing in of extra formatters and configuration to SimpleCov.

This was the simplest method of maintaining an existing SimpleCov configuration I could think of. Should close #25.

Example usage:

```ruby
CodeClimate::TestReporter.start(SimpleCov::Formatter::Console, SimpleCov::Formatter::HTMLFormatter) do
  add_filter "/config/"
  add_filter "/vendor/"
  add_filter "/features/"
  add_filter "/spec/"

  add_group "Authorizers", "app/authorizers"
  add_group "Controllers", "app/controllers"
  add_group "Helpers", "app/helpers"
  add_group "Libraries", "lib"
  add_group "Mailers", "app/mailers"
  add_group "Models", "app/models"
  add_group "Presenters", "app/presenters"
  add_group "Serializers", "app/serializers"
  add_group "Services", "app/services"
  add_group "Validators", "app/validators"
  add_group "Workers", "app/workers"
end
```

Let me know if I should change anything.