Use a PHPCS bootstrap file to auto-detect text-domain and minimum supported wp version

The I18n sniff needs a text_domain property to be set in a custom ruleset to trigger the text-domain checks.
The DeprecatedFunctions, DeprecatedClasses and the upcoming DeprecatedParameters (#826) sniffs need a minimum_supported_version property.

When the sniffs are used on plugins and themes, this information is typically already available in the theme/plugin headers in style.css/ readme.txt or the main plugin file and - at least for the minimum_supported_version a pain to keep in sync.

Since PHPCS 2.5.0 there appears to be a - rarely used - option to bootstrap a file to be run between when the sniffs are loaded and when they are actually run.
See:

• https://github.com/squizlabs/PHP_CodeSniffer/pull/783
• https://github.com/squizlabs/PHP_CodeSniffer/wiki/Advanced-Usage#using-a-bootstrap-file

I would like to suggest we explore the possibility of auto-detecting the above mentioned properties using a bootstrap file.

Some thoughts:

• Look only in the root of the directory being sniffed for any of the files mentioned & if the file is found, parse the top of the file similar to how WP core does. Looking further down or up can quickly become more complicated than is warranted.
• Custom text_domain properties if set should still be respected as this is an array and the property can be used to pass more than one text-domain.
  • So I suggest for the in-sniff logic to check for a custom property first and if none is found, use the auto-detected info. Alternatively, the auto-detected property could be merged into the custom property.
• Custom minimum_supported_version properties should be overruled if something different is auto-detected, as that is the "official" minimum supported version so this should always be leading.

I suggest this to be added to the Extra ruleset as these sniffs are typically contained therein.

Opinions ?

/cc @grappler This principle may also be useful for the theme sniffs.

Start with the I18n, DeprecatedFunctions, DeprecatedClasses, and upcoming DeprecatedParameters sniffs, the Extra ruleset, and PHPCS's bootstrap-file documentation. Review how style.css, readme.txt, and the main plugin file expose headers, then define the auto-detection behavior and how custom text_domain and minimum_supported_version properties should interact.