Excluded fenced code tags from being included in the output

I saw that fenced comments showed up in the plugin output. That messes up the TOC in my case. Excluded lines in fenced output from being interpreted as headlines.
I used [CommonMark](http://spec.commonmark.org/0.7/#fenced-code-blocks) as bases, but did not go into too much details, e.g. the number of fence-characters is not checked.