Reintroduce per directory configuration.

It's my opinion that a piece of useful functionality was lost in
https://github.com/bbatsov/rubocop/commit/d343279ed7bb0b13be2f97b97b627f6a50297bc9, namely that `.rubocop.yml` files can exist in several subdirectories and that they control the configuration for their directory trees.

This change brings back the old functionality.

This also means that "cops on duty" will mean all cops, including
the disabled ones, because we present it at the beginning and it can
potentially be a different list of cops in different directories.

Maybe you don't agree. Let's discuss.