Add option to display images stored online

If non-nil, this option will download images from a remote server (if
the URL is given with a white-listed protocol; by default only
allowing HTTPS) and display them during the execution of
`markdown-display-inline-images'.

Absolute process-specific caching is employed to avoid
double-downloading images.  No `nice' means is yet given for the
invalidation of this cache.

CHANGES.md

@@ -57,6 +57,7 @@
         table insertion.  ([GH-369][])
     -   Add `markdown-kill-outline` and `markdown-kill-block`
         functions.
+    -   Added options for viewing remote images.  ([GH-378][])
 
 *   Improvements:
 

markdown-mode.el

@@ -8449,6 +8449,33 @@ or \\[markdown-toggle-inline-images]."
   (mapc #'delete-overlay markdown-inline-image-overlays)
   (setq markdown-inline-image-overlays nil))
 
+(defcustom markdown-display-remote-images nil
+  "If non-nil, download and display remote images.
+See also `markdown-inline-image-overlays'.
+
+Only image URLs specified with a protocol listed in
+`markdown-remote-image-protocols' are displayed."
+  :group 'markdown
+  :type 'boolean)
+
+(defcustom markdown-remote-image-protocols '("https")
+  "List of protocols to use to download remote images.
+See also `markdown-display-remote-images'."
+  :group 'markdown
+  :type '(repeat string))
+
+(defvar markdown--remote-image-cache
+  (make-hash-table :test 'equal)
+  "A map from URLs to image paths.")
+
+(defun markdown--get-remote-image (url)
+  "Retrieve the image path for a given URL."
+  (or (gethash url markdown--remote-image-cache)
+      (let ((dl-path (make-temp-file "markdown-mode--image")))
+        (require 'url)
+        (url-copy-file url dl-path t)
+        (puthash url dl-path markdown--remote-image-cache))))
+
 (defun markdown-display-inline-images ()
   "Add inline image overlays to image links in the buffer.
 This can be toggled with `markdown-toggle-inline-images'
@@ -8466,24 +8493,29 @@ or \\[markdown-toggle-inline-images]."
               (end (match-end 0))
               (file (match-string-no-properties 6)))
           (when (and imagep
-                     (not (zerop (length file)))
-		     (file-exists-p file))
-            (let* ((abspath (if (file-name-absolute-p file)
-                                file
-                              (concat default-directory file)))
-                   (image
-                    (if (and markdown-max-image-size
-                             (image-type-available-p 'imagemagick))
-                        (create-image
-                         abspath 'imagemagick nil
-                         :max-width (car markdown-max-image-size)
-                         :max-height (cdr markdown-max-image-size))
-                      (create-image abspath))))
-              (when image
-                (let ((ov (make-overlay start end)))
-                  (overlay-put ov 'display image)
-                  (overlay-put ov 'face 'default)
-                  (push ov markdown-inline-image-overlays))))))))))
+                     (not (zerop (length file))))
+            (unless (file-exists-p file)
+              (when (and markdown-display-remote-images
+                         (member (downcase (url-type (url-generic-parse-url file)))
+                                 markdown-remote-image-protocols))
+                (setq file (markdown--get-remote-image file))))
+            (when (file-exists-p file)
+              (let* ((abspath (if (file-name-absolute-p file)
+                                  file
+                                (concat default-directory file)))
+                     (image
+                      (if (and markdown-max-image-size
+                               (image-type-available-p 'imagemagick))
+                          (create-image
+                           abspath 'imagemagick nil
+                           :max-width (car markdown-max-image-size)
+                           :max-height (cdr markdown-max-image-size))
+                        (create-image abspath))))
+                (when image
+                  (let ((ov (make-overlay start end)))
+                    (overlay-put ov 'display image)
+                    (overlay-put ov 'face 'default)
+                    (push ov markdown-inline-image-overlays)))))))))))
 
 (defun markdown-toggle-inline-images ()
   "Toggle inline image overlays in the buffer."