Allow sniffs to be ignored via user settings

This change allows users to specify a config option to ignore certain sniffs. PHP CodeSniffer doesn't allow this via the command link or we could just use `phpcs_additional_args`.

Example:

    "phpcs_ignore_sniffs": [
        "Drupal.Commenting.InlineComment.InvalidEndChar",
        "Drupal.Commenting.FunctionComment.ShortFullStop",
        "Drupal.Commenting.InlineComment.NotCapital"
    ]

When running PHPCS, any errors matching these sources will be ignored!