initial commit
Add h1, h2, br, a, and div; Refactor inline-block; Misc clean up

* Headings have interesting behaviour when splitting them and when
  collapsing a paragraph into it (select part of heading and paragraph and
  press backspace).

* <br>'s aren't very styleable by default, but can be made more
  styleable. Render them more visibly using content and :before as
  for inline elements.

* Add <div> to the whitelist since that is what contenteditable
  produces by default when pressing return after a heading (it
  doesn't create a paragraph).

* Add <a> to the whitelist.

* Reduce margin for inline elements (used to be inherited from the
  styles for block elements).
  Render the tagname for inline blocks before the element (instead
  of absolutely positioned to the top right, which caused them to
  overlap other inline text).

* Remove redundant [contenteditable] from <p>.

* Reset font better.