The delete-pair command from lisp.el deletes a matched pair of
delimiters.  After using this command, I often want to operate on the
region enclosed by the deleted delimiters.  As far as I know, there's
currently no quick way to access the end of that region.

This patch introduces a custom option, delete-pair-push-mark.  When
enabled, it causes delete-pair to push a mark at the end of the
enclosed region.

This opens up various workflows.  For instance, after deleting a pair,
we can use C-x C-x to highlight the region and act on it further.
Alternatively, we can use C-M-k to delete the first sexp inside the
delimiters, then act on the remainder.

To err on the side of caution, I hid the new behavior behind a user
option, disabled by default.  I could just as well see it being turned
on by default (or getting rid of the user option, making the new
behavior "mandatory"), and am open to suggestions/feedback.

Thanks, best,

Paul