Allow extensions to be overridden in contrib phpcs.xml file

Steps to reproduce:

- Clone coder.
- Create a folder called test_fails_folder
- Create a file called README.md with some errors in it. In my case it contains the following:

this is not a php file

```php
<?php

class Test {};
```

- Run phpcs with the following command:

$ ./vendor/bin/phpcs --standard=./coder_sniffer/Drupal --extensions=php,md test_fails_folder/

FILE: /xxx/coder/test_fails_folder/README.md
----------------------------------------------------------------------------
FOUND 3 ERRORS AFFECTING 1 LINE
----------------------------------------------------------------------------
 6 | ERROR | [ ] Class name doesn't match filename; expected "class README"
 6 | ERROR | [x] Missing class doc comment
 6 | ERROR | [ ] Closing class brace must be on a line by itself
----------------------------------------------------------------------------
PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
----------------------------------------------------------------------------

Time: 79ms; Memory: 8MB

This gives you errors.

Now try to remove the md extension:

./vendor/bin/phpcs --standard=./coder_sniffer/Drupal --extensions=php test_fails_folder/

As expected, you now get no errors.

Now, try to specify this in a config file:

<?xml version="1.0" encoding="UTF-8"?>
<ruleset name="myproject">
  <file>test_fails_folder</file>
  <arg name="extensions" value="php,md"/>
  <rule ref="./coder_sniffer/Drupal"></rule>
</ruleset>

And run phpcs with no arguments:

$ ./vendor/bin/phpcs 

FILE: /xxx/coder/test_fails_folder/README.md
----------------------------------------------------------------------------
FOUND 3 ERRORS AFFECTING 1 LINE
----------------------------------------------------------------------------
 6 | ERROR | [ ] Class name doesn't match filename; expected "class README"
 6 | ERROR | [x] Missing class doc comment
 6 | ERROR | [ ] Closing class brace must be on a line by itself
----------------------------------------------------------------------------
PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
----------------------------------------------------------------------------

Time: 82ms; Memory: 8MB

This now gives me an error. So let's try to remove md in the config:

<?xml version="1.0" encoding="UTF-8"?>
<ruleset name="myproject">
  <file>test_fails_folder</file>
  <arg name="extensions" value="php"/>
  <rule ref="./coder_sniffer/Drupal"></rule>
</ruleset>

$ ./vendor/bin/phpcs 

FILE: /xxx/coder/test_fails_folder/README.md
----------------------------------------------------------------------------
FOUND 3 ERRORS AFFECTING 1 LINE
----------------------------------------------------------------------------
 6 | ERROR | [ ] Class name doesn't match filename; expected "class README"
 6 | ERROR | [x] Missing class doc comment
 6 | ERROR | [ ] Closing class brace must be on a line by itself
----------------------------------------------------------------------------
PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
----------------------------------------------------------------------------

Time: 82ms; Memory: 8MB

I would expect this to not give me any errors, but it does, since Drupal ruleset overrides my config. With the patch attached, it does not give me any errors, as I expect