transient region-awareness

[Miles Bader]

The functionality embedded in `keep-lines-read-args' --

        * replace.el (keep-lines-read-args): New function.

seems like a good candidate for a new interactive spec, analogous to
`%r' (returning two arguments), but with the above functionality; `%R'
perhaps?

I'm not sure what you would call the lisp-function equivalents....
something like `operating-region-beginning' and `operating-region-end'?

-Miles