Set inferior-ess-prompt in stata's customize-alist

inferior-ess-prompt isn't getting built from
inferior-ess-primary-prompt and inferior-ess-secondary-prompt since
the code for that is in inferior-ess-mode and these aren't set up yet
when that gets called.

Closes #946 again (hopefully)

Author: jabranham

lisp/ess-stata-mode.el

@@ -79,6 +79,7 @@
     ;; --more-- is necessary here (hangs otherwise if startup stata.msg is big)
     (inferior-ess-primary-prompt   . "[.:] \\|--more--")
     (inferior-ess-secondary-prompt . "--more--")
+    (inferior-ess-prompt           . "\\([.:] \\|--more--\\)")
     (comint-use-prompt-regexp      . t)
     (inferior-ess-search-list-command   . "set more off\n search()\n")
     (ess-execute-screen-options-command . "set linesize %s\n")