Add rule subsets

Upon cloning the repo, you may run `bin/install-subsets` which will symlink each of the directories in the `Subsets` directory to the PHPCS standards directory, prefixed by `WordPress-`. For example, the `phpcs/CodeSniffer/Standards` directory would contain:

```
WordPress/
WordPress-All@ -> WordPress/Subsets/All
WordPress-Core@ -> WordPress/Subsets/Core
WordPress-Core-Extra@ -> WordPress/Subsets/Core-Extra
WordPress-Core-VIP@ -> WordPress/Subsets/Core-VIP
WordPress-Extra@ -> WordPress/Subsets/Extra
WordPress-VIP@ -> WordPress/Subsets/VIP
```

This then allows you to configure your project ruleset XML to reference one of these standards, for example:

```xml
<rule ref="WordPress-Core-Extra" />
```

This also makes it easy for PhpStorm to use one of these subset rules.

/cc @shadyvb