Expand folding support for all headers

[thesafetyguy]

Unless I'm wrong or missing something, H1, H2 and H3 headers will not fold. I try to keep a tidy hierarchy for my notes with proper header tiers rather than using them just for font size. Unfortunately, huge sections of my notes can't be folded because of this. Without a table of contents, it is difficult to get an overview of everything and hide things so they're not in the way.