[alextgordon]

This is pretty much what CKEditor does if you configure it to do that (simple toolbar, enter inserts <br>)

Thinking about it, the correct thing to do is to use <p> for each line, but allow the user to specify single or double spacing.

This is approximately what real word processors do, because often you will want one line to be left-aligned (e.g. to show English text) and the immediately next line to be right aligned (e.g. to show Hebrew text).