Add an external image to the test presentation.

Also, set `--fail-if-warnings` to fail on warnings.

example/Makefile

@@ -6,10 +6,10 @@ presentation.pdf: diagram.pdf
 presentation.pdf: prompt.gif
 
 %.pdf: %.md
-	pandoc -t beamer $< -o $@
+	pandoc --fail-if-warnings -t beamer $< -o $@
 
 left-to-right.pdf: left-to-right.md
-	pandoc --pdf-engine=xelatex $< -o $@
+	pandoc --fail-if-warnings --pdf-engine=xelatex $< -o $@
 
 %.pdf: %.dot
 	dot $< -Tpdf > $@

example/presentation.md

@@ -17,3 +17,6 @@ author: Your name here
 ## Animated gif
 
 ![Animated gif](prompt.gif)
+
+## External image
+![External image](https://www.shore.co.il/cgit/rcfiles/plain/.face)