feat: Add `Level::None` for omitting the header

[BurntSushi]

When combined with an empty title, this results in the header being omitted entirely.

This isn't what @epage suggested in #167 since changing our rendering to pass in a custom header is a bigger refactor than what I've been able to get to. The goal of this change was to be very small without requiring bigger code changes on our end. Unfortunately, this is a breaking change, so I'm not sure what the appetite is for this.

Fixes #167