org-clock.el: New function `org-clock-sum-custom'

org-clock.el: New function `org-clock-sum-custom'

* org-clock.el (org-clock-sum-today): Use `headline-filter'.
Remove interactive form.
(org-clock-display): Change the meaning of prefix argument:
one prefix arg shows today's time; two prefix args show clocks
from a custom range; three prefix args show the total time in
the echo area.
(org-clock-sum-custom): New function.
(org-clock-sum): Remove interactive form.