Add block_environment.html.haml to haml/html5 to accomodate the
environment block used in mathematical text, e.g.,

   [env.thml]
   --
   There are infinitely many prime numbers.
   --

I will push a similar block to erb/html5.  (It differs from the open
block only in its name - "environment" - and in that the content
is italicized, as in the theorem environment in LaTeX.  Note that
the part after "env." is arbitray: it can be "corollary", "definition",
"joke", etc.