Support URL rewriting for static sites

[rrthomas]

Static sites should be able to have their HTML files end .html. This is blocking #21.

[rrthomas]

Here's how we'll do it:

• Only rewrite URLs in static mode.
• Rewrite URLs in three places:
  • [x] The argument of $url, when the MIME type is one that is converted to HTML by default (see the list in line 653 of DarkGlass.pm, which is text/plain, text/markdown and text/x-readme).
  • [x] The output of the convert function, which will also output further conversions that need doing.
  • [x] The href of each a element in the raw HTML input (before expanding macros). In this latter case, simply rewrite it to a $url call, where it doesn't already start with a $ or a scheme. (An alternative would be to require that the user explicitly use $url, but that makes Markdown input nasty.)
  • [ ] Update static-site to call cv for the extra conversions.
• Rewrite by changing the file's extension to the canonical extension for the destination MIME type.