Add possibility to use inherit_from in .rubocop.yml.

This adds functionality for inherit_from 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,
and all configuration files inherit implicitly from config/default.yml.

Default values are not handled in code anymore, since full
configuration is always present.
Merge pull request #143 from jonas054/based_on

Add possibility to use inherit_from in .rubocop.yml.