Support specifying the image width in #+ATTR_HTML

Known issue as of Hugo v0.30: The Hugo figure shortcode does not
support specifying the height parameter! So ox-hugo will support only
the width parameter for now.

figure shortcode in Hugo source:
b6f2e3b438/tpl/tplimpl/template_embedded.go (L22-L37)

https://discourse.gohugo.io/t/can-i-find-out-the-image-size/2196/12?u=kaushalmodi

ox-hugo.el

@@ -949,7 +949,8 @@ and rewrite link paths to make blogging more seamless."
                               (class . ,(plist-get attr :class))
                               (attr . ,(plist-get attr :attr))
                               (attrlink . ,(plist-get attr :attrlink))
-                              (alt . ,(plist-get attr :alt))))
+                              (alt . ,(plist-get attr :alt))
+                              (width . ,(plist-get attr :width))))
              (figure-param-str ""))
         (dolist (param figure-params)
           (let ((name (car param))

test/site/content-org/all-posts.org

@@ -154,6 +154,24 @@ Some text before image.
 #+ATTR_HTML: :caption A unicorn!
 [[/images/org-mode-unicorn-logo.png]]
 Some more text, after image.
+*** Setting image size
+The image [[https://www.w3schools.com/tags/att_img_width.asp][width]] can be specified in *pixels* using the =:width=
+parameter. *The height of the image will be resized proportionally.*
+
+Below image is shown 50 pixels wide.
+#+ATTR_HTML: :width 50
+[[/images/org-mode-unicorn-logo.png]]
+
+Below image is shown 100 pixels wide.
+#+ATTR_HTML: :width 100
+[[/images/org-mode-unicorn-logo.png]]
+
+Below image is shown 1000 pixels width! Now the size of this image is
+200px × 200px. But the image will still show up in 1000px × 1000px
+size, but obviously heavily pixelated!
+
+#+ATTR_HTML: :width 1000
+[[/images/org-mode-unicorn-logo.png]]
 *** Other
 Similarly, =:link=, =:attr=, =:attrlink= parameters in =#+ATTR_HTML=
 are also supported to set the corresponding parameter in the Hugo

test/site/content/posts/figure-shortcode-and-attr-html.md

@@ -49,6 +49,26 @@ Some text before image.
 Some more text, after image.
 
 
+## Setting image size {#setting-image-size}
+
+The image [width](https://www.w3schools.com/tags/att_img_width.asp) can be specified in **pixels** using the `:width`
+parameter. **The height of the image will be resized proportionally.**
+
+Below image is shown 50 pixels wide.
+
+{{<figure src="/images/org-mode-unicorn-logo.png" width="50">}}
+
+Below image is shown 100 pixels wide.
+
+{{<figure src="/images/org-mode-unicorn-logo.png" width="100">}}
+
+Below image is shown 1000 pixels width! Now the size of this image is
+200px × 200px. But the image will still show up in 1000px × 1000px
+size, but obviously heavily pixelated!
+
+{{<figure src="/images/org-mode-unicorn-logo.png" width="1000">}}
+
+
 ## Other {#other}
 
 Similarly, `:link`, `:attr`, `:attrlink` parameters in `#+ATTR_HTML`