beautification via subprocess pipe and added pre-save hook

- call beautification via subprocess pipe, so no disk is touched
- added beautification of files as optional pre-save hook if run_on_save option is set (false by default for backward compatibility)
- make saving the file after running beautify_ruby command optional if save_after_beautify is set (true by default for backward compatibility)
- with pre-save hook no error is shown if focus is not a ruby file
- with pre-save hook no saving is done within the command to avoid infinite loop

if accepted, README should be amended slightly