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

doc/org.texi

@@ -6263,7 +6263,11 @@ be selected:
              thisyear, lastyear, thisyear-@var{N}     @r{a relative year}
              @r{Use @kbd{S-@key{left}/@key{right}} keys to shift the time interval.}
 :tstart      @r{A time string specifying when to start considering times.}
+             @r{Relative times like @code{"<-2w>"} can also be used.  See}
+             @r{@ref{Matching tags and properties} for relative time syntax.}
 :tend        @r{A time string specifying when to stop considering times.}
+             @r{Relative times like @code{"<now>"} can also be used.  See}
+             @r{@ref{Matching tags and properties} for relative time syntax.}
 :step        @r{@code{week} or @code{day}, to split the table into chunks.}
              @r{To use this, @code{:block} or @code{:tstart}, @code{:tend} are needed.}
 :stepskip0   @r{Do not show steps that have zero time.}
@@ -6314,6 +6318,11 @@ only to fit it into the manual.}
                     :tend "<2006-08-10 Thu 12:00>"
 #+END: clocktable
 @end example
+A range starting a week ago and ending right now could be written as
+@example
+#+BEGIN: clocktable :tstart "<-1w>" :tend "<now>"
+#+END: clocktable
+@end example
 A summary of the current subtree with % times would be
 @example
 #+BEGIN: clocktable :scope subtree :link t :formula %

lisp/org-clock.el

@@ -2604,9 +2604,9 @@ TIME:      The sum of all time spend in this tree, in minutes.  This time
       (setq te (format "%4d-%02d-%02d" (nth 2 te) (car te) (nth 1 te))))
     ;; Now the times are strings we can parse.
     (if ts (setq ts (org-float-time
-		     (apply 'encode-time (org-parse-time-string ts)))))
+		     (seconds-to-time (org-matcher-time ts)))))
     (if te (setq te (org-float-time
-		     (apply 'encode-time (org-parse-time-string te)))))
+		     (seconds-to-time (org-matcher-time te)))))
     (save-excursion
       (org-clock-sum ts te
 		     (unless (null matcher)