org-capture.el: Add support for week trees

* lisp/org-capture.el (org-capture-templates): Add
  file+weektree(+prompt) options.
  (org-capture-set-target-location): Add support for week trees.
* doc/org.texi (Template elements): Document file+weektree(+prompt)
  options.

doc/org.texi

@@ -7176,6 +7176,13 @@ one matched.}.
 @item (file+datetree+prompt "path/to/file")
 Will create a heading in a date tree, but will prompt for the date.
 
+@item (file+weektree "path/to/file")
+Will create a heading in a week tree for today's date.  Week trees are sorted
+by week and not by month unlike datetrees.
+
+@item (file+weektree+prompt "path/to/file")
+Will create a heading in a week tree, but will prompt for the date.
+
 @item (file+function "path/to/file" function-finding-location)
 A function to find the right location in the file.
 

etc/ORG-NEWS

@@ -167,6 +167,19 @@ for details.
 *** org-bbdb-anniversaries-future
 Used like org-bbdb-anniversaries, it provides a few days warning
 for upcoming anniversaries (default: 7 days).
+*** Support for ISO week trees
+ISO week trees are an alternative date tree format that orders entries
+by ISO week and not by month.
+
+For example:
+
+: * 2015
+: ** 2015-W35
+: ** 2015-W36
+: *** 2015-08-31 Monday
+
+They are supported in org-capture via ~file+weektree~ and
+~file+weektree+prompt~ target specifications.
 ** New functions
 *** ~org-show-children~ 
 It is a faster implementation of ~outline-show-children~.

lisp/org-capture.el

@@ -151,6 +151,12 @@ target       Specification of where the captured item should be placed.
              (file+datetree+prompt \"path/to/file\")
                  Will create a heading in a date tree, prompts for date
 
+             (file+weektree \"path/to/file\")
+                 Will create a heading in a week tree for today's date
+
+             (file+weektree+prompt \"path/to/file\")
+                 Will create a heading in a week tree, prompts for date
+
              (file+function \"path/to/file\" function-finding-location)
                  A function to find the right location in the file
 
@@ -331,6 +337,12 @@ you can escape ambiguous cases with a backward slash, e.g., \\%i."
 			  (list :tag "File & Date tree, prompt for date"
 				(const :format "" file+datetree+prompt)
 				(file :tag "  File"))
+			  (list :tag "File & Week tree"
+				(const :format "" file+weektree)
+				(file :tag "  File"))
+			  (list :tag "File & Week tree, prompt for date"
+				(const :format "" file+weektree+prompt)
+				(file :tag "  File"))
 			  (list :tag "File & function"
 				(const :format "" file+function)
 				(file :tag "  File    ")
@@ -908,21 +920,25 @@ Store them in the capture property list."
 	      (setq target-entry-p (and (derived-mode-p 'org-mode) (org-at-heading-p))))
 	  (error "No match for target regexp in file %s" (nth 1 target))))
 
-       ((memq (car target) '(file+datetree file+datetree+prompt))
+       ((memq (car target) '(file+datetree file+datetree+prompt file+weektree file+weektree+prompt))
 	(require 'org-datetree)
 	(set-buffer (org-capture-target-buffer (nth 1 target)))
 	(org-capture-put-target-region-and-position)
 	(widen)
-	;; Make a date tree entry, with the current date (or yesterday,
-	;; if we are extending dates for a couple of hours)
-	(org-datetree-find-date-create
+	;; Make a date/week tree entry, with the current date (or
+	;; yesterday, if we are extending dates for a couple of hours)
+	(funcall
+	 (cond
+	  ((memq (car target) '(file+weektree file+weektree+prompt))
+	   #'org-datetree-find-iso-week-create)
+	  (t #'org-datetree-find-date-create))
 	 (calendar-gregorian-from-absolute
 	  (cond
 	   (org-overriding-default-time
 	    ;; use the overriding default time
 	    (time-to-days org-overriding-default-time))
 
-	   ((eq (car target) 'file+datetree+prompt)
+	   ((memq (car target) '(file+datetree+prompt file+weektree+prompt))
 	    ;; prompt for date
 	    (let ((prompt-time (org-read-date
 				nil t nil "Date for tree entry:"