Inhibit read-only

When testing with ~Emacs -Q~ I noticed that the fringe space could not
be inserted because the prompt was read-only.
Add customization options, extra bitmaps

Allow users to specify how they would like the overlay images to look.
Also add 2 images in case people like these, I prefer the arrow myself.
Extract function for easier testing
Add URL header
Update docstring
Bump version