* TODO
** Allow collection pages to live in a different directory than posts.
** Feature request: Auto-reload pages when --watch detects changes
** External command processing
Hacky experimentation:
#+BEGIN_SRC scheme
  (use-modules (ice-9 match))

  (call-with-output-file "bloop.txt"
    (lambda (port)
      (match (primitive-fork)
        (0
         (close 1)
         (dup2 (fileno port) 1)
         (execlp "grep" "-e" "cat" "animals"))
        (pid
         (waitpid pid)))))

  (define (gcc c-file o-file)
    (list "gcc" "-o" o-file c-file))

  (define (external-artifact source destination command)
    (unless (file-exists? source)
      (error "input file does not exist" source))
    (make-artifact destination
                   (lambda (output)
                     (let ((command* (append command (list source))))
                       (format #t "run '~a' → '~a'~%"
                               (string-join command* " ")
                               destination)
                       (call-with-output-file output
                         (lambda (port)
                           ;; Run the command in a new process with
                           ;; stdout redirected into the output file.
                           (match (primitive-fork)
                             (0
                              (close 1)
                              (dup2 (fileno port) 1)
                              (apply execlp command*))
                             (pid
                              (match (waitpid pid)
                                ((_ . status)
                                 (unless (zero? (status:exit-val status))
                                   (error "command failed" command*))))))))))))

#+END_SRC