Add filter to center all images, even non-figure images (#153)

build.sh

@@ -497,6 +497,7 @@ if [ -n "${pdf_output}" -o -n "${latex_output}" ]; then
 		--filter=mermaid-filter
 		--lua-filter=informative-sections.lua
 		--lua-filter=convert-images.lua
+		--lua-filter=center-images.lua
 		--lua-filter=parse-html.lua
 		--lua-filter=apply-classes-to-tables.lua
 		--lua-filter=landscape-pages.lua

filter/center-images.lua

@@ -0,0 +1,11 @@
+function Image (img)
+    -- Center images even if they aren't captioned.
+    if FORMAT:match 'latex' then
+        return {
+            pandoc.RawInline('latex', '\\begin{center}'),
+            img,
+            pandoc.RawInline('latex', '\\end{center}'),
+        }
+    end
+    return img
+end

guide.tcg

@@ -553,13 +553,24 @@ becomes:
 
 ![Adding an Image](add_plus_button.jpg){#fig:add-plus-button width=60%}
 
-The `{#fig:add-plus-button}` attribute (note there are no spaces between the `)` and the `{`!) does two things:
+Including a figure title (`Adding an Image` between the `[]` above) causes the image to numbered, included in
+the list of figures, and given a caption.
 
-1. Includes the figure in the List of Figures (if you used `\listoffigures` as described in @sec:toc).
-2. Numbers the figure so you can reference it as @fig:add-plus-button by just typing `@fig:add-plus-button`.
+The `{#fig:add-plus-button}` attribute (note there are no spaces between the `)` and the `{`!) allows you to
+reference it as @fig:add-plus-button by just typing `@fig:add-plus-button`.
 
 Including `width=60%` here specifies that the image should take up 60% of the page's width.
 
+Without the title and `#fig` attribute,
+
+```md
+![](add_plus_button.jpg)
+```
+
+looks like:
+
+![](add_plus_button.jpg)
+
 ![An SVG Image](stack.svg){#fig:sample-svg width=50%}
 
 Even formats that are not natively supported by Pandoc are now supported, like the SVG in @fig:sample-svg.