A simple, stop-gap, right-to-left mode toggle

[eyalroz]

Adding proper right-to-left support to a markdown app, or engine, is not trivial.

However - a relatively simple feature to implement is to implement a toggle in the web editor UI, which simply flips the direction of the entire document - in the editor widget and in the output. It's wrapping with a <div> and changing an attribute of its style; or not even wrapping - just changing something in the relevant top-level div or DOM element.

This could be a prelude to #1029 .