feat: Support non-sectionize headings, enclosed by equal number of hashes

[MurakamiShinyu]

This adds the following heading auto-sectioning rule:

• Do not sectionize if the heading line starts with #s and ends with equal or greater number of #s.
  • ### Not Sectionize ### (enclosed by equal number of #s) -- not sectionize
  • ### Sectionize ## (insufficient number of closing #s) -- sectionize

resolves #155