Adding Reverse Existence checks Issue #1334

[vqle23]

I added a way to ensure that you only words within a certain set as requested in Issue #1334 or as the reporter coin as a "reverse-existence check". Currently, the two sets implemented with this new feature are the top 1000 words set as requested by the initial reporter, but I also added a set of words that elementary kids are taught to learn how to spell. With both sets, I have provided a suite of test cases for quality assurance. However, I did have to modify test_config_flag.py a bit to pass linter tests (some lines were too long, and had to turn a string into a variable on the line above).