Headings add exit matcher for heading

Paragraphs add exit matcher for elements (but it should be sans paragraph)




* foo
* bar
* baz

context tree -> ()

match * foo

context tree -> exit(heading matcher)

check exit
invoke heading matcher
check exit
invoke heading matcher
check exit
invoke heading matcher
adds second heading matcher exit


Ways around this:
- Always parse SOMETHING before checking for exit
  - Doesn't always seem possible
- Disable exit matchers during exit check
  - Seems like it would break syntax
- Have separate parsers for the beginning of the exit condition (for example, checking for just the headline instead of the full heading parser)
  - Won't be possible with paragraphs ending at any other element
- Check exit matchers in parent parser
  - Will this work? seems like it would just create larger loops