Add rule awesome/heading

[itaisteinherz]

The awesome/heading rule lints all of the given file's headings, verifying that:

• There is exactly one heading.
• It's in title case.
• It's children array is flat (i.e. it doesn't have any nested and complicated child elements).

Fixes #59

[transitive-bullshit]

This PR generally looks solid @itaisteinherz.

In addition to my small inline comment, I'd like to fix #75 before merging just to be sure that it won't have any unintended side effects.

[itaisteinherz]

@transitive-bullshit Any chance you could take another look at this? 🙃

[sindresorhus]

I'd like to fix #75 before merging just to be sure that it won't have any unintended side effects.

Ideally, I would agree, but #75 is still not fixed and it cannot block all other work forever.