org.el: Allow %(my-function) as a specifier in abbreviated links.

* org.el (org-link-expand-abbrev): Implement "%(my-function)"
as a new specifier.  Update the docstring.

* org.texi (Link abbreviations): Illustrate the use of the
"%h" specifier.  Document the new "%(my-function)" specifier.

Thanks to Takaaki ISHIKAWA who came all the way down from Tokyo
to Paris and raised a not-so-distant issue: "could we translate
emails from the mailing list and have a URI for each translated
email?"  See the update in the manual for an answer.

doc/org.texi

@@ -3570,18 +3570,26 @@ that relates the linkwords to replacement text.  Here is an example:
 @smalllisp
 @group
 (setq org-link-abbrev-alist
-  '(("bugzilla" . "http://10.1.2.9/bugzilla/show_bug.cgi?id=")
-    ("google"   . "http://www.google.com/search?q=")
-    ("gmap"     . "http://maps.google.com/maps?q=%s")
-    ("omap"     . "http://nominatim.openstreetmap.org/search?q=%s&polygon=1")
-    ("ads"      . "http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?author=%s&db_key=AST")))
+  '(("bugzilla"  . "http://10.1.2.9/bugzilla/show_bug.cgi?id=")
+    ("url-to-ja" . "http://translate.google.fr/translate?sl=en&tl=ja&u=%h")
+    ("google"    . "http://www.google.com/search?q=")
+    ("gmap"      . "http://maps.google.com/maps?q=%s")
+    ("omap"      . "http://nominatim.openstreetmap.org/search?q=%s&polygon=1")
+    ("ads"       . "http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?author=%s&db_key=AST")))
 @end group
 @end smalllisp
 
 If the replacement text contains the string @samp{%s}, it will be
-replaced with the tag.  Otherwise the tag will be appended to the string
-in order to create the link.  You may also specify a function that will
-be called with the tag as the only argument to create the link.
+replaced with the tag.  Using @samp{%h} instead of @samp{%s} will
+url-encode the tag (see the example above, where we need to encode
+the URL parameter.)  Using @samp{%(my-function)} will pass the tag
+to a custom function, and replace it by the resulting string.
+
+If the replacement text don't contain any specifier, it will simply
+be appended to the string in order to create the link.
+
+Instead of a string, you may also specify a function that will be
+called with the tag as the only argument to create the link.
 
 With the above setting, you could link to a specific bug with
 @code{[[bugzilla:129]]}, search the web for @samp{OrgMode} with

lisp/org.el

@@ -1319,9 +1319,12 @@ The 'linkkey' must be a word word, starting with a letter, followed
 by letters, numbers, '-' or '_'.
 
 If REPLACE is a string, the tag will simply be appended to create the link.
-If the string contains \"%s\", the tag will be inserted there.  Alternatively,
-the placeholder \"%h\" will cause a url-encoded version of the tag to
-be inserted at that point (see the function `url-hexify-string').
+If the string contains \"%s\", the tag will be inserted there.  If the string
+contains \"%h\", it will cause a url-encoded version of the tag to be inserted
+at that point (see the function `url-hexify-string').  If the string contains
+the specifier \"%(my-function)\", then the custom function `my-function' will
+be invoked: this function takes the tag as its only argument and must return
+a string.
 
 REPLACE may also be a function that will be called with the tag as the
 only argument to create the link, which should be returned as a string.
@@ -8668,6 +8671,8 @@ call CMD."
 	  (setq rpl (cdr as))
 	  (cond
 	   ((symbolp rpl) (funcall rpl tag))
+	   ((string-match "%(\\([^)]+\\))" rpl)
+	    (replace-match (funcall (intern-soft (match-string 1 rpl)) tag) t t rpl))
 	   ((string-match "%s" rpl) (replace-match (or tag "") t t rpl))
 	   ((string-match "%h" rpl)
 	    (replace-match (url-hexify-string (or tag "")) t t rpl))