revised optional Gregorian validity verification

- added `org-node-ensure-gregorian-validity` defcustom - Nb: left it off by default - but if on, `org-node--example-daily-whereami` implements additional checks on both "NNNN-NN-NN" basenames and also on attempted `org-node-datestamp-format` putative dates to verify that they're valid Gregorian dates [as per [previous discussion](https://github.com/meedstrom/org-node/pull/31), probably not *really* necessary, but it seemed like an interesting problem to solve, and this is at least less ugly than `^\\(?:[0-9][0-9]\\)\\{1,2\\}-\\(?:1[012]\\|0?[1-9]\\)-\\(?:0?[1-9]\\|[12][0-9]\\|3[01]\\)$`.] (same as [other pull request](https://github.com/meedstrom/org-node/pull/33), but forgot to put in a branch)