I had a look around, but I couldn't find an option to toggle or add a vertical ruler in the editor.

It would be great to be able to toggle one or more vertical rulers at user defined page widths (e.g., at 80, 100, however many characters).   Ideally, the rulers should be rendered as unobtrusive, thin lines, duller than the copy text. 

Hi,

Yeah, there’s no character ruler implemented currently. I’ll add it to the “features I’d like to add” list, but not ETA on that currently :)