Add multiple levels of violation to the style checker

[ryangyoung]

It would be nice to have both "critical" style violations and "advisory" style violations, or maybe even more levels. So the build would only break on critical errors, but you could take "suggestions" from the other guidelines.

Also it would be nice to be able to mark specific cases as exceptions to style rules.

If this works, we could add style checking to CI.

Original jira ticket: https://ripplelabs.atlassian.net/browse/DOC-372