Allow relative paths to be used in --config-set installed_paths

This patch adds the ability to use relative paths in `installed_paths` directive, to reference *subset* Standards' packages added inside existing Standards.

This is backward compatible, paths that begins with a slash will still be parsed normally, but those without a preceding slash will be prefixed with the path to Standards directory ( `/path/to/phpcs/CodeSniffer/Standards` ), so using the following:
```bash
phpcs --config-set installed_paths WordPress/Subsets
```
will cause PHPCS to detect WordPress/Subsets/%subset-name%/ruleset.xml

The use-case for this is discussed at https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/pull/162, and it is clear that changes in this PR would save Standards packages much effort trying to work-around the need to figure out absolute paths, which proves difficult working with multi-environment setup like with VVV.

/cc @westonruter @Rarst @gsherwood, insights are welcome!