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/
Since all that is changing is the tag name, do we really need to repeat the entire line again?