When the user types a heading using the Markdown preset, the heading is a larger font. <h1>s are larger than <h6>s.
Fixes a bug with the AppKit editor where typing regular paragraph text two lines after a heading would cause the insertion point to appear large, then small.
Updates Markdown preset test snapshots for large headings
New feature requested in #55
<h1>s are larger than<h6>s.