Improve get_section_headings on check script

**Work in progress. Don't merge it.**

The check script doesn't get headings of type

~~~
> Call out {.callout}
~~~

or

~~~
> Challenge {.challenge}
~~~

For #104 we need that the check script get this type of headings.

This pull request changes the check script so it now gets all type of headings but this change breaks `_validate_section_heading_order`.

## TODO

- [ ] Refactor `_validate_section_heading_order`.
- [ ] Fix `_validate_section_heading_order`.