Specify customize variable :type attribute

Author: syohex

inf-clojure.el

@@ -450,7 +450,8 @@ number (e.g. (\"localhost\" . 5555))."
    "Form to be used to start `inf-clojure'.
 Can be a cons pair of (host . port) where host is a string and
 port is an integer, or a string to startup an interpreter like
-\"planck\".")
+\"planck\"."
+   :type '(choice (cons string integer) (const nil)))
 
 (defcustom inf-clojure-custom-repl-type
   nil