Several changes with varying degrees of obvious significance; all Reek-inspired.

- Broke out new `Meldd::Internals::KeywordFormatter` class from `ArticleDecorator#format_keywords`, with full spec coverage.
- ArticleDecorator#format_keywords now uses the newly-broken-out `Meldd::Internals::KeywordFormatter` class to do its "dirty work".
- Finally broke utility and decorator classes out into their own tabs in coverage report.
- Reek, by default, only allows one "nested" iterator; i.e., no nesting at all. That is changed in our `defaults.reek` to 2.