SES with case insensitive cell names for jumping.

SES with case insensitive cell names for jumping.

* doc/misc/ses.texi (The Basics): Document that ses-jump may be
customized.
(Customizing @acronym{SES}): Document new customisations for ses-jump.

* lisp/ses.el (ses-jump-cell-name-function)
(ses-jump-prefix-function): New defcustoms.
(ses-jump-prefix): New defun.
(ses-jump): Make ses-jump use the new defcustoms.

* test/lisp/ses-tests.el (ses-jump-B2-prefix-arg)
(ses-jump-B2-lowcase, ses-jump-B2-lowcase-keys)
(ses-jump-B2-symbol, ses-jump-B2-renamed): New tests.