Function: open-rectangle

Blank out the region-rectangle, shifting text right.

The text previously in the region is not overwritten by the blanks,
but instead winds up to the right of the rectangle.

When called from a program the rectangle's corners are START and END.
With a prefix (or a FILL) argument, fill with blanks even if there is
no text on the right side of the rectangle.
(fn START END &optional FILL)