lib/pry.rb,lib/pry/cli.rb: fix the "no-prompt" switch

This sets two prompts for Pry. The issue is that if you have a custom
prompt (like me), then you'll face this error:

  undefined method `[]=' for #<Proc:0x00000002f329e0>

The problem is that the no-prompt switch doesn't set the second prompt.
Also, be consistent with the codebase and store the prompt in a
constant.

lib/pry.rb

@@ -70,6 +70,8 @@ class Pry
   # A simple prompt - doesn't display target or nesting level
   SIMPLE_PROMPT = [proc { ">> " }, proc { " | " }]
 
+  NO_PROMPT = [proc { '' }, proc { '' }]
+
   SHELL_PROMPT = [
                   proc { |target_self, _, _| "#{Pry.config.prompt_name} #{Pry.view_clip(target_self)}:#{Dir.pwd} $ " },
                   proc { |target_self, _, _| "#{Pry.config.prompt_name} #{Pry.view_clip(target_self)}:#{Dir.pwd} * " }

lib/pry/cli.rb

@@ -167,7 +167,7 @@ Copyright (c) 2013 John Mair (banisterfiend)
   end
 
   on "no-prompt", "No prompt mode" do
-    Pry.config.prompt = proc { '' }
+    Pry.config.prompt = Pry::NO_PROMPT
   end
 
   on :r, :require=, "`require` a Ruby script at startup" do |file|