disallow trailing whitespace on empty lines

We've flip-flopped on this issue. Recently in #182 @antonkovalyov expressed interest in a patch disallowing (again) trailing whitespace on otherwise empty lines. I'm not sure whether this breaking change is acceptable. Ugly as it might be, we could appease both sides by supporting *three* values for `trailing`: `false`, `true`, and `"strict"`. At this stage I'm submitting the simplest possible change. I will update the PR in response to feedback.