Making HTML more wysiwyg

[John L. Clark]

The attached patch adds support in our HTML exporter for our margin
sizes and "hitting enter twice to add a new line", which while a
horrendous behaviour, is still used by everyone, including everyone who
wrote our docs in .../abi/docs.  :)

I will be working on supporting more of our document characteristics as
well as putting some serious work in on the styles dialog in the near
future.

Take care,

        John