Add possibility to use based_on in .rubocop.yml.

Part of solution for #116. This is a pretty big change. I expect some discussion.

This adds functionality for `based_on` and uses it in the new
`config/*.yml` files, and also from our own `.rubocop.yml`.

Default when no `.rubocop.yml` is found is to use `config/default.yml`.

Nothing is changed regarding default handling in code, and printing
of warning messages for missing parameters has not been implemented.