org-contacts: Fix org-contacts-gnus-article-from-goto

* contrib/lisp/org-contacts.el (org-contacts-filter): Add a PROP-MATCH
argument.
(org-contacts-gnus-article-from-get-marker): Search for email as a
property not a tag.

`org-contacts-gnus-article-from-get-marker' seems to assume that the
contact properties exist within the contact's tags as colon-separated
KEY={VALUE} entries.  This isn't the case for me, using org-mode from
git master.

This commit adds a PROP-MATCH argument to `org-contacts-filter' so
that `org-contacts-gnus-article-from-get-marker` can filter by the
EMAIL property.

contrib/lisp/org-contacts.el

@@ -233,10 +233,15 @@ A regexp matching strings of whitespace, `,' and `;'.")
       (progress-reporter-done progress-reporter)))
     org-contacts-db))
 
-(defun org-contacts-filter (&optional name-match tags-match)
-  "Search for a contact matching NAME-MATCH and TAGS-MATCH.
-If both match values are nil, return all contacts."
+(defun org-contacts-filter (&optional name-match tags-match prop-match)
+  "Search for a contact matching any of NAME-MATCH, TAGS-MATCH, PROP-MATCH.
+If all match values are nil, return all contacts.
+
+The optional PROP-MATCH argument is a single (PROP . VALUE) cons
+cell corresponding to the contact properties.
+"
   (if (and (null name-match)
+	   (null prop-match)
 	   (null tags-match))
       (org-contacts-db)
     (loop for contact in (org-contacts-db)
@@ -244,6 +249,11 @@ If both match values are nil, return all contacts."
 	      (and name-match
 		   (org-string-match-p name-match
 				       (first contact)))
+	      (and prop-match
+		   (org-find-if (lambda (prop)
+				  (and (string= (car prop-match) (car prop))
+				       (org-string-match-p (cdr prop-match) (cdr prop))))
+				(caddr contact)))
 	      (and tags-match
 		   (org-find-if (lambda (tag)
 				  (org-string-match-p tags-match tag))
@@ -523,7 +533,8 @@ A group FOO is composed of contacts with the tag FOO."
          (email (cadr address)))
     (cadar (or (org-contacts-filter
                 nil
-                (concat org-contacts-email-property "={\\b" (regexp-quote email) "\\b}"))
+		nil
+                (cons org-contacts-email-property (concat "\\b" (regexp-quote email) "\\b")))
                (when name
                  (org-contacts-filter
                   (concat "^" name "$")))))))