Relative paths

A correction for the `Include` and `Exclude` parameters that any cop can have.

In a file named `.rubocop.yml`, the paths are relative to the directory where the configuration file is. In a configuration file with a different name, such as `default.yml`, the paths are relative to the `.rubocop.yml` file inheriting from it, or if there's no such file, the current directory.

Fixes #892.