Feature/1.x/42/toc block find all headings

[rupertj]

This PR will:

• Split out the part of the ToC block that finds headings in content into its own HeadingFinder service. This makes it easier for us to test, but also lets people substitute their own implementation if they want to.
• Add an interface for HeadingFinder.
• Make the HeadingFinder more flexible in the headings it can find, compared to the existing code.
• Add unit test coverage for the HeadingFinder.

Fixes #42

[finnlewis]

Nice! I think merge this @rupertj - I was just flagging it to @j4-m for interest as much as anything

[rupertj]

Cool. Will do. Thanks!