Use Markdown to make email -> blog conversion easy

**Important**: before this goes live, need to make sure that the redcarpet gem is installed.

The current email format works pretty well as Markdown, just need to recognize that a single line break isn't enough to cause a line break in the rendered output (e.g., the Recap author's signature) and switching to markdown headers improves the output.

Even if old emails are just pasted into a new .md file (with the header block added) and no changes, the output should be acceptable.