Improve file diff logger

I have improved the following two points.

 - Execution time.

Added the state to FileDiffLogger whether already been executed once.

- Added the command-line option and  configuration

FileDiffLogger is disabled by default, but the following will be enabled.

`phync --file-diff /path/to/file`

```php
<?php
// config.php
return array(
    'destinations' => array(
        'foo.example.com',
    ),
    'file_diff' => true, // It is false if it is omitted.
);
```