R1:1494476d304b

Style <h1> -> <h6> for Remarkup

Summary:
most users will notice this makes h1 and h2 bigger.

my design algorithm was to start with h6 and make that the
size of regular text and then gently scale upwards to the mighty
h1. i used margins so things would collapse nice and first-child /
last-child so there wouldn't be any longer-than-planned spacing.

Test Plan:
made a few docs like

= header =

== sub header ==

=== sub sub header ===

(etc)

in phriction and they looked good to me. made some comments
in…