Introduce a way to tweak warning/error level for checks

[beaufour]

I've just run the linter on our puppet manifests and we have a fair amount of trailing_whitespace-check, but I wouldn't consider it an error. Ideally I'd like to turn most checks into warnings for now, and then when we get our code cleaned up a bit I can turn things into errors again (as they would/should be breaking our CI setup)

[rodjek]

If you don't particularly care about trailing whitespace, you should just disable that check (--no-trailing_whitespace)

[beaufour]

Ah no, I don't want to disable them. I'd like to turn them into warnings, so I still see them, but they shouldn't be fatal.