Allow inverse `existence_check`s (re: up-goer-five check)

[patcon]

Hi there! This tool is neat. Thanks!

I was hoping to create a check for raising issue when any words outside a set were used: (top 1000 english words) https://splasho.com/upgoer5/phpspellcheck/dictionaries/1000.dicin

Can you suggest the simplest way to go about this? I'm happy to submit a PR when I get around to it :)

Inspiration: https://splasho.com/upgoer5/