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 differential like that and they looked good to me.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1094