Allow difficult_words method to return the set, and add en_uk dictionary

[Niall47]

I wanted to use the list of difficult words rather than just seeing the count so I've added an optional argument for that. I added an rspec test for that.

I also converted the en_us dictionary to en_uk

[sonarqubecloud[bot]]

Quality Gate passed

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[Niall47]

Thanks. Any chance you can cut a release to RubyGems?

[kupolak]

@Niall47 yes, I'll do it today after work

[kupolak]

https://rubygems.org/gems/textstat/versions/0.1.9 @Niall47