v0.5.0

Articles imported from another system now come back exactly as they went in, and the preview is
sent the cover image and the rest of the frontmatter.

• Lists keep their spacing. A list with a blank line between its items — the way turndown and
  so most imported articles write them — was saved as a tight one, which the site draws closer
  together. It stays loose now, and keeps the spaces after its markers (- ), so it goes back
  byte for byte. A rule keeps its spelling too (* * *).
• Extra blank lines are no longer an empty paragraph. Three blank lines in a row were read as a
  blank line of their own and saved as  , adding space to the page that was never there.
  They are one blank line now, which is all the site ever made of them.
• The preview is sent the cover image and the rest of the frontmatter: article.image, as an
  address the page can load even before it is uploaded, and article.fields. A preview page
  written for 0.4.0 ignores them and works as before.