Add the ability to specify mode in Rex output file

* Because sometimes you might want to append
* Preserves original hardcoded 'wb' as default
* http://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html

lib/rex/ui/text/output/file.rb

@@ -14,8 +14,8 @@ class Output::File < Rex::Ui::Text::Output
 
   attr_accessor :fd
 
-  def initialize(path)
-    self.fd = ::File.open(path, "wb")
+  def initialize(path, mode='wb')
+    self.fd = ::File.open(path, mode)
   end
 
   def supports_color?