resolves #369 make level 0 section titles more prominent in TOC

- put level 0 section titles in their own level
- add class to each nested outline that cooresponds to the section level
- default stylesheet updates:
* make level 0 section titles italic
* align level 0 and level 1 section titles
* add margin around level 1 section nested in a level 0 section
- add additional toc tests
Merge pull request #370 from mojavelinux/toc-classes

resolves #369 make level 0 section titles more prominent in TOC