resolves #393 numbering should not increment on unnumbered sections

- set state of numbered flag on section when creating it
- track section number in addition to positional index
- use section number to build outline number (i.e., sectnum)
- number special sections by default
- add more tests
Merge pull request #398 from mojavelinux/section-numbering

resolves #393 numbering should not increment on unnumbered sections