We were following it about half of the time and we even added it to our
former `.mdlrc` file. However, for some reason, MDL doesn't detect this
rule when specified in the `.mdlrc` file, so we didn't notice we weren't
following it in many cases.
Now that we're using a style file to configure MDL, we can enable this
rule again and apply it, since now MDL correctly includes it in its
report.
This rule is broken when we have ".,;::!?" in the headers.
We have removed the ones with ":" but kept the ones
with "?", so this rule will always warn in these cases.
