Add support for additional escape characters

Hi Dmitri,

Small change here to add support for the additional prescribed escape characters as per http://daringfireball.net/projects/markdown/syntax#backslash.

Unit tests passing.

Actually while it does get the job done, I think the current control character escaping is technically wrong. As currently implemented, `(md-to-html-string "\\*")` gives `"<p>&#42;</p>"` where I believe the spec's intention is actually just to give `"<p>*</p>"`. It's not a serious problem. I'm assuming you did this intentionally as an easy way of implementing the control character escaping?

One idea: maybe we could run the process back again after the markdown transforms to replace the `&#;` encoded forms with their original characters? That way you could keep the same [simple] implementation, if that was a goal. As I said, nothing urgent - just thinking out-loud.

Thanks a lot, cheers! :-)