Section headings

I implemented the arbitrary section headings the way I think they should look like. I also added a test case and updated the other test cases.

Please have a look at my changes and tell me what you think of them.

In a nutshell:
* Section headings are marked as `@par Section`.
* New paragraphs are considered part of the section if their indentation is larger than that of the section heading.

Related to issue #7