Allow installed_paths to be relative to the phpcs root directory

The `installed_paths` config expects the paths to be absolute. This causes problems in an environment like Vagrant, where PHPCS and a repo of standards is located in a synced folder, since the absolute path to the other standards would be different from the inside of Vagrant than from outside of Vagrant. 

If a relative path *is* supplied, there is an additional challenge because any relative `installed_paths` are interpreted to be relative of the current working directory, not the directory where PHPCS is installed.

This PR makes relative `installed_paths` relative to where PHPCS is installed. 

It also adds support for `installed_paths` relative to the user's home directory if the path begins with `~/`.