eww: Feature request: Dont hard-wire shr-external-rendering-functions

[raman@google.com]

I have a feature request motivated by the need to "tag" portions of
the EWW rendered buffer with the tag-name of the sub-tree that
produced the rendering; this helps me then implement structured
navigation of the EWW buffer for Emacspeak.
I've achieved this for header tags etc via an advice on the relevant shr-tag-<tagname> functions.



At present the <article> element  is handled by shr-generic which is a defsubst -- so advice wont work.
 I implemented byself a simple emacspeak-eww-tag-article that does:
(defun emacspeak-eww-tag-article (dom)
  "Tag article, then render."
  (let ((start (point)))
    (shr-generic dom)
        (put-text-property start (point) 'article 'eww-tag)))

But at present, I have no way of telling EWW to add this to shr-external-rendering-functions -- since that variable is let-bound in eww-display-html --- I could probably still get away with advicing shr-insert-document -- but I'd   prefer a simpler solution:-)

Possibilities --

1. Implement shr-tag-article  as above -- essentially rename emacspeak-eww-tag-article to shr-tag-article  -- and update the other shr functions in the recursive descent walk of the HTML tree to set property 'eww-tag -- 

2. If this is overkill for mainstream Emacs users -- then perhaps allow external packages to customize what list of rendering functions eww uses?

For completeness, here is how I generate the advice on the shr-tag-<name> functions to emit the necessary eww-tag properties in the EWW rendered buffer:

(loop
 for  tag in
 '(h1 h2 h3 div                    ; sectioning
      ul ol dl                     ; Lists
      li dt dd p                   ; block-level: bullets, paras
      form blockquote              ; block-level
      a b it em span               ; in-line
      br hr                        ; separators
      th tr table )
 do
 (eval
  ` (defadvice  ,(intern (format "shr-tag-%s" tag)) (around emacspeak pre act comp)
    (let ((start (point)))
      ad-do-it
      (let ((start (if (char-equal (following-char) ?\n)
                       (min (point-max) (1+ start) )start))
            (end (if (> (point) start) (1- (point)) (point))))
        (put-text-property start end
                           (quote ,tag) 'eww-tag)
        (when (memq (quote ,tag) '(h1 h2 h3))
          (put-text-property start end 'h 'eww-tag)))))))

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