Support style-guide only usage

Many cops that have been implemented and enabled by default have no
correlation to a style-guide rule: `Metrics/ClassLength`,
`Metrics/AbcSize`, `Metrics/PerceivedComplexity` just to name a few. In
cases where these cops find violations, there's little recourse as the
rules cannot be traced back to a meaningful discussion or consensus.

The `--only-guide-cops` and `AllCops/StyleGuideCopsOnly` options give
users an easy method of invoking RuboCop in a way that's more congruent
with the rules of the style guide.
Can `cop_config` really be `nil`?