Use '=' instead of '#' for atx-style headers in markdown+lhs.

Using atx-style headers in markdown+lhs doesn't give a legal literate haskell file, since the usage of '#' is reserved.  I suggest supporting '=' instead of '#' for headings of level 3 or more.  This should pose no compatibility problems, since '#' is already illegal, and the '=' syntax is not used for anything else.  Using html style subheaders works only when generating html, not for other backends.