Allow option for -s (simpify)

[Kubuxu]

gofmt allows to specify -s option to also flag code that can be simplified. It would be nice to be able to use it via .codeclimate.yml.

[dblandin]

@Kubuxu We'd happily review a PR for that!

Looks like the change would be to update the invoked command here based on the config content: https://github.com/codeclimate-community/codeclimate-gofmt/blob/master/codeclimate-gofmt.go#L29