Allow regexp separator when converting the region to a table

* lisp/org-table.el (org-table-convert-region): Interpret string SEPARATOR
as regular expression.  Triple `C-u' prefix arg will read a regexp
from the user.
* doc/org.texi: Document that `C-c |' can take a regexp as a separator.

Patch modified from a proposal by Francois.

doc/org.texi

@@ -2143,7 +2143,8 @@ TAB character, the function assumes that the material is tab separated.
 If every line contains a comma, comma-separated values (CSV) are assumed.
 If not, lines are split at whitespace into fields.  You can use a prefix
 argument to force a specific separator: @kbd{C-u} forces CSV, @kbd{C-u
-C-u} forces TAB, and a numeric argument N indicates that at least N
+C-u} forces TAB, @kbd{C-u C-u C-u} will prompt for a regular expression to
+match the separator, and a numeric argument N indicates that at least N
 consecutive spaces, or alternatively a TAB will be the separator.
 @*
 If there is no active region, this command creates an empty Org

lisp/org-table.el

@@ -547,7 +547,9 @@ following values:
 
 '(4)     Use the comma as a field separator
 '(16)    Use a TAB as field separator
+'(64)    Prompt for a regular expression as field separator
 integer  When a number, use that many spaces as field separator
+regexp   When a regular expression, use it to match the separator
 nil      When nil, the command tries to be smart and figure out the
          separator in the following way:
          - when each line contains a TAB, assume TAB-separated material
@@ -557,6 +559,8 @@ nil      When nil, the command tries to be smart and figure out the
   (let* ((beg (min beg0 end0))
 	 (end (max beg0 end0))
 	 re)
+    (if (equal separator '(64))
+	(setq separator (read-regexp "Regexp for field separator")))
     (goto-char beg)
     (beginning-of-line 1)
     (setq beg (point-marker))
@@ -591,6 +595,8 @@ nil      When nil, the command tries to be smart and figure out the
 		 (if (< separator 1)
 		     (user-error "Number of spaces in separator must be >= 1")
 		   (format "^ *\\| *\t *\\| \\{%d,\\}" separator)))
+		((stringp separator)
+		 (format "^ *\\|%s" separator))
 		(t (error "This should not happen"))))
       (while (re-search-forward re end t)
 	(replace-match "| " t t)))