Add difficulty level

[Errolyn]

Add difficulty levels.

Easy: word length 4-7 letters; not using letters xz
Medium: word length 7-10 letters; not using xz
Hard: word length 7-13; no letter limitations
Extreme: word length > 13 letters; no letter limitations

[Errolyn]

The random word API will allow you to choose a word length [len]. For each of the difficulty options there will need to be logic added to randomly pick a word length each time, then the incoming word will need to be checked for letters that should be avoided.

It might be a good idea to get a few words right up front to help avoid repeated calls that could slow down the code. Then have logic to see if any of the words in the array meet the difficulty rules.