Add the site-posts-output-directory field.

- Remove the "prefix" argument from the theme objects in the blog
  builder.
- Remove the "blog-prefix" argument from the rss and atom builders.

A user can specify the output directory for the posts in the SITE
record, this, together with the SITE-SLUG function, allows one to
generate relative and absolute URLs to the posts. This information is
used in all the builders that operate on posts collections.

website/haunt.scm

@@ -117,11 +117,7 @@ culture works available under the " ,%cc-by-sa-link " license.")))))))
                  " — " ,(date->string* (post-date post)))
              (div ,(post-sxml post))))
          #:collection-template
-         (lambda (site title posts prefix)
-           (define (post-uri post)
-             (string-append "/" (or prefix "")
-                            (site-post-slug site post) ".html"))
-
+         (lambda (site title posts)
            `(,(jumbotron
                `((p "Haunt is a simple, functional, hackable static site
 generator written in Guile Scheme that gives authors the ability to
@@ -164,7 +160,7 @@ without needing to upload the generated files to a web server.")
              (ul
               ,@(map (lambda (post)
                        `(li
-                         (a (@ (href ,(post-uri post)))
+                         (a (@ (href ,(site-post-path site post)))
                             ,(post-ref post 'title)
                             " — "
                             ,(date->string* (post-date post)))))