Added a derived backend for org export

See web-stacker for the background.

yantar92 on #org-mode pointed out that a derived backend would be a cleaner solution. I had initially thought it was too complicated, but I have to agree the example in the org-mode documentation does pretty much what I need.

This new approach has the big advantage that the generation of URLs happens at export time, so it's not possible for the displayed program code and the version encoded in the URL to get out of sync.

;; derived backend to customize src block handling
(defun my-beamer-src-block (src-block contents info)
  "Transcode a SRC-BLOCK element from Org to beamer
         CONTENTS is nil.  INFO is a plist used as a communication
         channel."
  (let ((attr (org-export-read-attribute :attr_latex src-block :stacker)))
    (concat
     (when (or (not attr) (string= attr "both"))
       (org-export-with-backend 'beamer src-block contents info))
     (when attr
       (let* ((body  (org-element-property :value src-block))
              (table '(? ?\n ?: ?/ ?? ?# ?[ ?] ?@ ?! ?$ ?& ??
                         ?( ?) ?* ?+ ?, ?= ?%))
              (slug (org-link-encode body table))
              (simplified (replace-regexp-in-string "[%]20" "+" slug nil 'literal)))
         (format "\n\\stackerlink{%s}" simplified))))))

(defun my-beamer-export-to-latex
    (&optional async subtreep visible-only body-only ext-plist)
  "Export current buffer as a (my)Beamer presentation (tex).
    See org-beamer-export-to-latex for full docs"
  (interactive)
  (let ((file (org-export-output-file-name ".tex" subtreep)))
    (org-export-to-file 'my-beamer file
      async subtreep visible-only body-only ext-plist)))

(defun my-beamer-export-to-pdf
    (&optional async subtreep visible-only body-only ext-plist)
  "Export current buffer as a (my)Beamer presentation (PDF).
  See org-beamer-export-to-pdf for full docs."
  (interactive)
  (let ((file (org-export-output-file-name ".tex" subtreep)))
    (org-export-to-file 'my-beamer file
      async subtreep visible-only body-only ext-plist
      #'org-latex-compile)))

(with-eval-after-load "ox-beamer"
  (org-export-define-derived-backend 'my-beamer 'beamer
    :translate-alist '((src-block . my-beamer-src-block))
    :menu-entry '(?l 1 ((?m "my beamer .tex" my-beamer-export-to-latex)
                        (?M "my beamer .pdf" my-beamer-export-to-pdf)))))

An example of using this in an org-document would as below. The first source code block generates only a link in the output while the last adds a generated link to the normal highlighted source code.

* Stuff
** Frame
#+attr_latex: :stacker t
#+NAME: last
#+BEGIN_SRC stacker :eval no
  (f)
#+END_SRC

#+name: smol-example
#+BEGIN_SRC stacker :noweb yes
  (defvar x 1)
  (deffun (f)
    (let ([y 2])
      (deffun (h)
        (+ x y))
      (h)))
  <<last>>
#+END_SRC

** Another Frame 
#+ATTR_LATEX: :stacker both
#+begin_src smol :noweb yes
  <<smol-example>>
#+end_src