test, docs: Add a linter for testing markdown, and fix up our docs

While writing docs, I couldn't remember what the correct style was
supposed to be, and I remember someone complaining about this
previously, so I decided to add a linter! I excluded a bunch of annoying
style rules, but if we find more we can add those to the list too.

Hopefully this gives us a more consistent feel throughout.

test/util.sh

@@ -18,7 +18,7 @@ fi
 
 fail_test()
 {
-	echo "FAIL: $@"
+	echo -e "FAIL: $@"
 	exit 1
 }