Investigate if all uses of difficult_words are correct

[alxwrd]

As reported in #70 and fixed in #71, a difficult word is: not in the easy_word_set and more than one syllable.

This is not true for Gunning Fog, and might not be for other calculations that rely upon difficult_words(). Need to verify the other formulas that use difficult_words.

[TiredFalcon]

The Dale-Chall readability formula does not use syllables counts to identify difficult words, but only the easy_words list.

Therefore, the count of easy words for Dale-Chall should be computed with:

count = word_count - self.difficult_words(text, syllable_threshold=0)

[alxwrd]

@TiredFalcon thank you for this! 🎉 I've raised #127.

[dogweather]

Note also, difficult_words() only returns the count of distinct difficult words, not the total in the text. IMO this is a bug and has been giving wrong results: #192 #193