Add support for \\ in Markdown

The double backslash is essentially the LaTeX equivalent of an HTML
break tag, but it wasn't being passed through the pipeline properly
from Markdown.

[Delivers #62685096]
Issue pull request for branch doublebackslash-62685096

[Delivers #62685096](https://www.pivotaltracker.com/story/show/62685096)
[#62685096]

* doublebackslash-62685096:
  Issue pull request for branch doublebackslash-62685096
  Add support for \\ in Markdown