Add exists command

This command shows whether or not the given url is known to CLark.

lisp/clark.lisp

@@ -140,6 +140,17 @@ BM should be a list containing the url and name of the bookmark."
       (insert-bookmark url name description)
       (add-tags tags))))
 
+(defcommand exists
+    "Check if a bookmark exists in the database."
+  (if (execute-single *db* "SELECT rowid FROM bookmark WHERE url = ?"
+                      (car args))
+      (progn
+        (format t "yes~%")
+        0)
+      (progn
+        (format t "no~%")
+        1)))
+
 (defcommand help
     "Show help message."
   (declare (ignore args))