This is not a missing feature in Jerboa, it's a design choice in the Markdown syntax. It's done so that one can break up long lines in the .md file without affecting the rendered page. Markdown is a standard, and Jerboa uses an existing tool to format posts. In order to make this work for Jerboa the devs would have to break compatibility with Markdown and create their own rendering tool. They're most likely not going to do it, and I don't think they should.

That's not a problem, though, because you can already create single line breaks in Jerboa, using standard Markdown. All you have to do is add two spaces at the end of your first line, where you want your line break to be. So, if I write down:

This is a line<space><space>
This is another line

this gets rendered to:

This is a line
This is another line

There are other ways to create line breaks in Markdown:

• Using an HTML <br/> tag
• Using a backslash \

but they're not supported by all renderers. For example: the <br/> tag works in Jerboa, but not in the web UI. Double space works for me in both.