Allow relative times in clocktable tstart and tend options

Allow relative times in clocktable tstart and tend options

* lisp/org-clock.el (org-clock-get-table-data): Pass tstart and tend
time strings through `org-matcher-time' to allow relative times besides
absolute ones, convert result to encoded time.
* doc/org.texi (The clock table): Document acceptance of relative times
in tstart and tend, link to syntax description and provide example.

Inspired in the original relative times proposal by Ilya Shlyakhter,
this is less invasive and it doesn't modify core functions, thus it
avoids the original's infinite recursion when hitting normal dates in
other locations.

TINYCHANGE