Improve formatting for Markdown.

We try to keep our Markdown rendered output as close as possible to the
source text formatting, but there's only so much that can be done by
CSS. This change massively improves the rendering by introducing a set
of new Markdown processors that clean up newlines and attributes, giving
us the output our users would expect.

This primarily does the following:

* Keeps the correct number of blank lines between blocks (by default,
  Python Markdown will just trim them away).

* Keeps code blocks (and certain other extension-provided blocks) from
  having an unnecessary extra blank line after the HTML (interfering
  with our <pre> formatting).

* Allows line breaks in lists.

* Wrapped lines of text inside an ordered list item no longer shows the
  wrapped text at the same indentation level as the item's counter
  value. It now properly stays within the confines of the list item.

  To make this happen, we had to use CSS counters and some custom
  styles (using `display: table` and friends).

* Allows ordered lists to start at a particular value. We do this by
  turning off lazy_ol, which gives us a start= attribute on the list.
  Then, to get it to play nice with the custom counters mentioned above,
  we have a Markdown block processor that adds CSS to the <ol> that
  initializes the counter to the right value.

* Adds unit tests to ensure these formatting rules do not break in
  the future.

The result is much nicer formatting for most cases (all cases I've
tested).

Reviewed at https://reviews.reviewboard.org/r/6643/