New features:
* Markdown parser can now create entry previews. Simply include `<​!-- more -->` in the text of an entry and use the `:preview` key instead of `:contents`.
* Text substitution middleware (`substitute`) with optional Clojure evaluation (`substitute-by`) - use `{​{key-name-or-Clojure-code}}` in your entries, and substitution middleware will... Substitute it for you. Yay?