New heading structure

Aims to create a more hierarchical heading structure. Somewhat based on structure in Twenty Fifteen and inspired by http://blog.rrwd.nl/2014/11/21/html5-headings-in-wordpress-lets-fight/ and https://www.joedolson.com/2014/11/headings-hierarchy-problem/
This is how it's don't historically. It is verbose, but also easier to understand. Twenty Fifteen and several other default themes use some variant of the same syntax.