Add option to use external command for initial HTML to Markdown conversion

As you suggested on [reddit](http://www.reddit.com/r/emacs/comments/2d3fxy/write_gmail_in_emacs_the_easy_way_gmailmessagemode/cjm414k), I modified `ham-mode` so that you can use an external command to do the initial HTML to Markdown conversion or use your `html-to-markdown` package to do it. It defaults to `pandoc` when `pandoc` is found on the path, but there's no good reason for it not to default to `html-to-markdown` always.