Add Thor.deprecation_warning

This reverts commit a88ea56792c9a40d803f025ffcb63e8ae35bf5ff.

Author: marcandre

lib/thor.rb

@@ -339,6 +339,13 @@ def disable_required_check?(command) #:nodoc:
       command && disable_required_check.include?(command.name.to_sym)
     end
 
+    def deprecation_warning(message) #:nodoc:
+      unless ENV['THOR_SILENCE_DEPRECATION']
+        warn "Deprecation warning: #{message}\n" +
+          'You can silence deprecations warning by setting the environment variable THOR_SILENCE_DEPRECATION.'
+      end
+    end
+
   protected
 
     def stop_on_unknown_option #:nodoc: