Add ability to get a random word from the entire corpus

[schmich]

Right now, random words must be picked from a specific category (noun, adjective, ...). There should be a way to get a random word across all categories.

• Support class and instance methods: Spicy::Proton.word, gen.word
• Support constraints: gen.word(min: 2, max: 5)
• Support formatters: Spicy::Proton.format('%w %w %w')
• Support word list: gen.words gives everything in the corpus
• Update readme