Major redesign for minimal rebuild. Bump to v0.18

- Use a new .frog/build file to cache results of the previous build.

  It is relatively expensive to parse a markdown file and enhance its
  HTML with Pygments etc. Likewise evaluating a Scribble file. In some
  cases, the resulting HTML can often be reused as-is -- merely
  "re-flowed" through post-template.html and page-template.html.

  Also this can help us detect when a source file has been deleted, or
  a tag is no longer used by any post, and therefore the associated
  output files may be deleted.

- Use file timestamps to detect other dependencies.

As a result, the common case of adding a new blog post results in
simply needing to:

- generate the new page

- update any index pages and feeds for the tag(s) used by the new post

- adjust the previously-newest post -- since it's "newer post" link
  must change, by re-flowing its pre-parsed HTML through post-template
  and page-template.

Likewise, adding or removing a tag for a post simply results in
rebuilding that post, plus the index pages and feeds associated with
the tag.

A full build is no faster than before, but a full rebuild now rarely
happens.