Subset rules with PHPCS mod.

Work on this branch is carried out while having `subset-selection` [branch] (https://github.com/x-team/PHP_CodeSniffer/tree/subset-selection) of https://github.com/x-team/PHP_CodeSniffer checked out.

Usage of rulesets is simplified now by using the following syntax:
```
phpcs --standard=WordPress:core
```

Subset rulesets also appear in `phpcs -i` so it can be used within PHPStorm and alike.

/cc @westonruter