set null prompt function in dumbterm mode

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: k1complete

lib/iex/lib/iex/cli.ex

@@ -53,13 +53,18 @@ defmodule IEx.CLI do
     if tty_works? do
       :user_drv.start([:"tty_sl -c -e", tty_args])
     else
-      :user.start
+      :application.set_env(:stdlib, :shell_prompt_func, 
+                           {__MODULE__, :prompt})
+      :user.start()
       unless match? {:win32, _}, :os.type do
         IO.puts "Warning: could not run smart terminal, falling back to dumb one"
       end
       local_start()
     end
   end
+  def prompt(_n) do
+    []
+  end
 
   # Check if tty works. If it does not, we fall back to the
   # simple/dumb terminal. This is starting the linked in