[Refactor] Use PathUtil in more places

Remove duplicate definitions of match_path?() and relative_path().
[Fix #892] Make Include/Exclude paths relative to .rubocop.yml

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.
Merge pull request #905 from jonas054/relative_paths

Relative paths