Introduce `Finstyle.config` to determine the path to YAML config file.
Remove redundant inheritance of enabled/disabled from default.yml.

The upstream.yml (which comes from RuboCop) already inherits enabled.yml
and disabled.yml and thus was being inherited from twice.
Add `finstyle-config` which outputs the full path to config YAML.

This is useful if invoked from the command line as input to another
command. For example:

    rubocop -D --config $(finstyle-config)