Subset selection

This allows usage of subset rulesets within a Standard directory, so you can add multiple rulesets, like `core.ruleset.xml` in root of your Standard, and pick it up on the command line via something like 
```
phpcs --standard=MyStandard:core
```

This also shows subsets registered in `phpcs -i` so it can be used by PhpStorm and others who query installed standards in their configuration.

Why we did this, is to allow usage of different subset of rules within a single Standard package, not all of people would agree to all rules within a standard package, and this makes it easier to provide pre-selected set of rules to users, usable instantly after installing of a Standard.

We've been struggling with this issue for a while at [WordPress-Coding-Standards] (https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards) for a while, with specific two cases in mind:
- Having a VIP set of rules that only applies to WordPress VIP environment
- Having a set of rules that only applies to a WordPress Theme context

This should be 100% backward compatible.