Add single line break option

This feature is useful for ensuring that lots of extra line breaks do not end up in the resulting Markdown file in situations like Evernote .enex exports, where notes look like this: 

    <?xml version="1.0" encoding="UTF-8" standalone="no"?>
    <!DOCTYPE en-note SYSTEM "http://xml.evernote.com/pub/enml2.dtd">
    <en-note>
    <div>Here's a line.</div>
    <div><br /></div>
    <div>Here's another line.</div>
    </en-note>

Note that this only works properly if body-width is set to 0.