Allow headings ending in question marks in markdown

Not sure whether it's a good idea to end a heading with a question mark.
When in doubt, we usually keep the code as it is now, so that's what
we're doing.

.mdl_style.rb

@@ -7,3 +7,4 @@ exclude_rule "MD041"
 exclude_rule "MD046"
 
 rule "MD007", indent: 2
+rule "MD026", punctuation: ".,;:!"