Differentiate Remarkup header sizes more clearly

Summary:
Ref PHI1275. Previously, see T591. See also T7963. Headers are currently very visually similar to one another, and similar to the text size:

I think the design intent was to make it hard to make bad-looking documents, but all the headers end up being very samey.

Differentiate the sizes of the headers better so they're much more obvious (e.g., when scrolling through a document) and the different levels are more distinct.

This might be a little overboard, but we can always pull it back a bit if it's too much, and I think giving users more control in Remarkup (in cases where it doesn't create some weird syntax/parsing nightmare) is generally a good thing.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20569