Added 404 and 403 page support, custom content type

- Added new content type, "custom", which leaves all content up to the content
  file by rending only the content itself (no titles, dates, etc). The
  standard head and sidebar are still included.
- Added 404 and 403 pages (content/404.md and content/403.md), which are of
  type "custom". These pages have their urls set as /hidden/404 and /hidden/403.
  The /hidden directory is intended as a directory for files which will not be
  accessed directly, but will instead by used by the web server. So, for
  example, with /hidden/404 and /hidden/403, the server running synful.io is
  configured to use them as the pages to render when the relevant errors
  occur. That server is also configured to treat the tree rooted at /hidden as
  nonexistent; requests for any of those resources will result in 404 errors.
- Added apache-conf, which is a copy of the site configuration file installed
  on the server running synful.io, albeit with some cruft removed and certain
  private details anonymized. Notably, besides the basic setup common to all
  Apache site of this form, the configuration includes directives to set up
  the 404 and 403 pages mentioned above, and to prevent the tree rooted at
  /hidden from being accessible through web requests.
Added support for Disqus comments.

- Added disqus.html fragment, which renders the Disqus widget (so long as the
  parameter "no_disqus" is not set to true or some equivalent). The code is a
  slightly modified version of the code that Disqus provides themselves.
  Specifically, it adds the condition that the widget not be loaded if the
  site is being accessed through the localhost domain. This is because Disqus
  uses page loads to register its discussion threads, so simply loading the
  page with a url such as http://localhost:1313/post/my-post would be enough
  to create a discussion thread for that url.
- Modified the single post template to include this Disqus widget.
- Added the Disqus shortname as a site-wide parameter.