org-clock: Add `untilnow' time block
* lisp/org-clock.el (org-clock-special-range): Handle `untilnow'
range.
(org-clock--oldest-date): New variable.
(org-clock-display-default-range): Add `untilnow' as valid range. Use
new variable.
(org-clock-display): Offer `untilnow' as a possible range.
* doc/org.texi (The clock table): Document `untilnow'
* testing/lisp/test-org-clock.el (test-org-clock-clocktable-contents-at-point): Fix test
when called interactively.
(test-org-clock/clocktable):
(test-org-clock/clocktable1): Update tests.
(test-org-clock/clocktable-until-now): New test.
* etc/ORG-NEWS (title): Document new feature.
Suggested-by: Sébastien Vauban