Add possibility to provide a list of excluded files or variables

[bogdannbv]

Depends on #113

This can (should?) be done similar to what phpunit or phpcs do with their configuration files (phpcs.xml and phpunit.xml).

We can have a desniff.yml file that can look something like this:

excluded_files:
    - ./config/view.php
    - ./config/queue.php

excluded_vars:
    # config/app.php
    - ASSET_URL

   # config/view.php
    - VIEW_COMPILED_PATH

   # config/broadcasting.php
    - PUSHER_APP_KEY
    - PUSHER_APP_SECRET
    - PUSHER_APP_ID
    - PUSHER_APP_CLUSTER