Strip internal comments from generated markdown.

    <!--- triple dashed comment is stripped -->
    <!-- double dashed comment is passed through -->

I based this on the loose idea in:

   https://groups.google.com/forum/#!topic/pandoc-discuss/FnTRTIhCEi4

This seeks to avoid a problem with layout of the home page.
After publishing the 2.11.0 announcement, the presence of a
multiline HTML comment at the start of the *previous* news
announcement was enough to result in a wonky layout!