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

[ZedThree]

I have a practically identical change in #159 for very similar reasons :)

[BurntSushi]

Ah nice, whoops, missed that!

[ndmitchell]

I'm also after pretty much the same change. Any chance this could be reviewed?

[Muscraft]

I am sorry for not reviewing or commenting on this before now. annotate-snippets is currently in the middle of a major rewrite, as part of the project goal to use it for rustc's output. This has been my primary focus over the past few months. That said, I plan to add support for this when the new version is released (hopefully in the next few weeks).

[epage]

With v0.12, we now have Level::no_name which I believe resolves this concern