html: Support links with image descriptions

Turns out Org mode supports image links natively and we don't have to go out of
spec!

From https://orgmode.org/manual/Images-in-HTML-export.html:

[...] if the description part of the Org link is itself another link, such as
‘file:’ or ‘http:’ URL pointing to an image, the HTML export back-end in-lines
this image and links to [...]

org/html_writer.go

@@ -341,16 +341,26 @@ func (w *HTMLWriter) WriteRegularLink(l RegularLink) {
 	if prefix := w.document.Links[l.Protocol]; prefix != "" {
 		url = html.EscapeString(prefix) + strings.TrimPrefix(url, l.Protocol+":")
 	}
-	description := url
-	if l.Description != nil {
-		description = w.WriteNodesAsString(l.Description...)
-	}
 	switch l.Kind() {
 	case "image":
-		w.WriteString(fmt.Sprintf(`<img src="%s" alt="%s" title="%s" />`, url, description, description))
+		if l.Description == nil {
+			w.WriteString(fmt.Sprintf(`<img src="%s" alt="%s" title="%s" />`, url, url, url))
+		} else {
+			description := strings.TrimPrefix(String(l.Description), "file:")
+			w.WriteString(fmt.Sprintf(`<a href="%s"><img src="%s" alt="%s" /></a>`, url, description, description))
+		}
 	case "video":
-		w.WriteString(fmt.Sprintf(`<video src="%s" title="%s">%s</video>`, url, description, description))
+		if l.Description == nil {
+			w.WriteString(fmt.Sprintf(`<video src="%s" title="%s">%s</video>`, url, url, url))
+		} else {
+			description := strings.TrimPrefix(String(l.Description), "file:")
+			w.WriteString(fmt.Sprintf(`<a href="%s"><video src="%s" title="%s"></video></a>`, url, description, description))
+		}
 	default:
+		description := url
+		if l.Description != nil {
+			description = w.WriteNodesAsString(l.Description...)
+		}
 		w.WriteString(fmt.Sprintf(`<a href="%s">%s</a>`, url, description))
 	}
 }

org/inline.go

@@ -383,6 +383,14 @@ func isValidPostChar(r rune) bool {
 func isValidBorderChar(r rune) bool { return !unicode.IsSpace(r) }
 
 func (l RegularLink) Kind() string {
+	description := String(l.Description)
+	descProtocol, descExt := strings.SplitN(description, ":", 2)[0], path.Ext(description)
+	if ok := descProtocol == "file" || descProtocol == "http" || descProtocol == "https"; ok && imageExtensionRegexp.MatchString(descExt) {
+		return "image"
+	} else if ok && videoExtensionRegexp.MatchString(descExt) {
+		return "video"
+	}
+
 	if p := l.Protocol; l.Description != nil || (p != "" && p != "file" && p != "http" && p != "https") {
 		return "regular"
 	}

org/testdata/inline.html

@@ -76,6 +76,9 @@ not emphasized/</p>
 <p>regular link to https (image) <img src="https://placekitten.com/200/200#.png" alt="https://placekitten.com/200/200#.png" title="https://placekitten.com/200/200#.png" /></p>
 </li>
 <li>
+<p>regular link with image as description <a href="https://placekitten.com"><img src="https://placekitten.com/200/200#.png" alt="https://placekitten.com/200/200#.png" /></a></p>
+</li>
+<li>
 <p>regular link enclosed in [] [<a href="https://www.example.com">https://www.example.com</a>] [<a href="https://www.example.com">example.com</a>]</p>
 </li>
 <li>

org/testdata/inline.org

@@ -28,8 +28,9 @@
   4. regular link to a file (video) [[my-video.mp4]]
   5. regular link to http (image) [[http://placekitten.com/200/200#.png]]
   6. regular link to https (image) [[https://placekitten.com/200/200#.png]]
-  7. regular link enclosed in [] [[[https://www.example.com]]] [[[https://www.example.com][example.com]]]
+  7. regular link with image as description [[https://placekitten.com][https://placekitten.com/200/200#.png]]
-  8. auto link, i.e. not inside =\[[square brackets]\]= https://www.example.com
+  8. regular link enclosed in [] [[[https://www.example.com]]] [[[https://www.example.com][example.com]]]
+  9. auto link, i.e. not inside =\[[square brackets]\]= https://www.example.com
 - timestamps
   - <2019-01-06>
   - <2019-01-06 Sun>

org/testdata/inline.pretty_org

@@ -28,8 +28,9 @@
   4. regular link to a file (video) [[my-video.mp4]]
   5. regular link to http (image) [[http://placekitten.com/200/200#.png]]
   6. regular link to https (image) [[https://placekitten.com/200/200#.png]]
-  7. regular link enclosed in [] [[[https://www.example.com]]] [[[https://www.example.com][example.com]]]
+  7. regular link with image as description [[https://placekitten.com][https://placekitten.com/200/200#.png]]
-  8. auto link, i.e. not inside =\[[square brackets]\]= https://www.example.com
+  8. regular link enclosed in [] [[[https://www.example.com]]] [[[https://www.example.com][example.com]]]
+  9. auto link, i.e. not inside =\[[square brackets]\]= https://www.example.com
 - timestamps
   - <2019-01-06 Sun>
   - <2019-01-06 Sun>