Add support for escaped line breaks

[Witiko]

jgm#42 implements Pandoc's escaped line breaks. Perhaps we can rebase their implementation on witiko/markdown? In Pandoc, escaped line breaks are the only way to create hard line breaks in multiline and grid table cells. In witiko/markdown, we cannot type trailing spaces in a TeX document in any format except ConTeXt, so this could be a useful alternative syntax.