Improve generated HTML for module directive

[cgay]

The HTML generated for the module directive (:mod:`foo`) looks exactly like the HTML for other Dylan language entities. Given that we will normally use .. module:: foo to start a module reference section I think we have two choices:

1. Generate a section header for the module:

   The Foo Module

2. Just create an anchor to reference but leave the section header to be created manually. I'm thinking this might be necessary because we don't know what section level to use. (i.e., what character to use for underlining)

[cgay]

I didn't check, but I wonder if .. library:: has the same problem.