--diff option and diff output

Allows to do --dry-run --verbose --diff to preview changes made in each file.

Example:

```
➜  PHP-CS-Fixer git:(color_diff) ✗ php php-cs-fixer fix --verbose --config=sf21 --dry-run --diff ../testproj
   1) TestProj/Resources/config/services.yml (eof_ending)
---------- begin diff ----------
--- Original
+++ New
@@ @@
             - [ setDoctrine, [ @doctrine ] ]
+

---------- end diff ----------

   2) TestProj/DependencyInjection/Configuration.php (return)
---------- begin diff ----------
--- Original
+++ New
@@ @@
         // more information on that topic.
-
         return $treeBuilder;
     }
 }
 

---------- end diff ----------

   3) TestProj/Listener/IpListener.php (return, eof_ending)
---------- begin diff ----------
--- Original
+++ New
@@ @@
                 $event->stopPropagation();
+

@@ @@
 }
+