Feature: new Block type: Separator

[horacioh]

Problem

A horizontal line or separator line to divide sections in a document.

Based on #57

[ericvicenti]

I think sections are already divided sufficiently with headings and hierarchy, right? If we want a horizontal separator, I'm not sure it should be a separate block type. Probably a good idea to use a <hr> between every h1 section

Or if we REALLY want publishers to control the separator for each block, it can be an attribute of headings. (but I think its not ideal! Right now, blocks all represent important content for the document, and a Separator is purely cosmetic and not really important to understand the content) It could also be a setting for the reader (and web site)