Bug fix, improvement and new feature

Hi,

* Improve `file_diff_previous` command with selection

Now a diff can be performed with a selection both in the previous and current file.


* Fix bug on clipboard command

There was a bug with clipboard diff and untitled file.


* Add `apply_tempfile_changes_after_diff_tool` feature

This option enables to apply the changes made from temp files which are creating during the external tool diff step, to the corresponding buffer (file or clipboard).