mu4e: default to mu4e-shr2text when available

Make shr the default renderer for rich-text emails, if shr is
available. Update the documentation.

mu4e/mu4e-contrib.el

@@ -51,25 +51,6 @@
 
 ;;;
 
-(defun mu4e-shr2text ()
-  "Html to text using the shr engine; this can be used in
-`mu4e-html2text-command' in a new enough emacs. Based on code by
-Titus von der Malsburg."
-  (interactive)
-  (let ((dom (libxml-parse-html-region (point-min) (point-max)))
-	 ;; When HTML emails contain references to remote images,
-	 ;; retrieving these images leaks information. For example,
-	 ;; the sender can see when I openend the email and from which
-	 ;; computer (IP address). For this reason, it is preferrable
-	 ;; to not retrieve images.
-	 ;; See this discussion on mu-discuss:
-	 ;; https://groups.google.com/forum/#!topic/mu-discuss/gr1cwNNZnXo
-	(shr-inhibit-images t))
-    (erase-buffer)
-    (shr-insert-document dom)
-    (goto-char (point-min))))
-
-
 ;;; Bookmark handlers
 ;;
 ;;  Allow bookmarking a mu4e buffer in regular emacs bookmarks.