Highlight markdown syntax in .md files using Redcarpet+CodeRay

Currently, there are something like three ways we do syntax highlighting in this app:

In .step files, you can have a 'source_code' step that specifies
a language parameter. This makes a <pre class='code'> tag that
contains the language name as ':::ruby', to be scooped out by
Rack::Codehighlighter.

In .deck.md files, you can use Github-flavored "```ruby" style
'fenced code blocks', which get parsed by redcarpet into <pre><code>
blocks, which inexplicably and briefly have an '@@@ ruby' directive
that is picked up by the Rack::Codehighlighter middleware.

In .md files (as of this diff) you can do Github-flavored fenced
code blocks, which are parsed in the original render time into
<pre class='CodeRay'> blocks that have the real syntax highlighting.

'Fenced code blocks' produced in Markdown-aware erector functions
(like 'message') don't seem to produce highlighted code. Will
probably improve on this in a subsequent diff.