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.
I saw a similar `nil` check in [`Config#for_cop`](../blob/v0.27.1/lib/rubocop/config.rb#L74) but I'm happy to remove it. There seems to be a lack of checks elsewhere, though mainly in cop implementation where you can safely assume configuration should exist.