Support "soft-return" atom (impossible due to github.com/jbarone/mobiledoc)

[tv42]

Mobiledoc atom soft-return means the thing you usually insert with shift+enter, a line break but not a paragraph break.

github.com/jbarone/mobiledoc has an API that is unable to support line breaks as atoms.
It expects an atom to convert to a string, and then puts that string through whitespace trimming.
Markdown linebreaks are \\\n or <space><space>\n, with significant whitespace in both.

Writing this down here just to leave a note.