Add flexible game logic

[gmittal]

It would be really cool if people could create their own word-based games using this as a base platform. Currently, the game is simply a turn-based game against the computer where each player enters a letter and avoids creating an OOV word (as in dsfhasdf, not an incomplete word that is capable of being completed or turned into an in-vocabulary word) or finishing a real word.

[gmittal]

I want to be able to play games like the "animal" or "country/city" game that I used to play on long road trips with my family, where players take turns naming entities that start with the last letter of the previous player's entity. There also can't be any repeats, and I think that's interesting stuff to experiment with, because it forces people to think of more than just a shortlist of ten cities that cover many of the frequent letter endings.

[gmittal]

Currently there is only one parameter that can be changed in the game logic, and that's the length of the minimum valid word. Initially, if you typed the letter a, the game would complete almost instantly because there are lots of two letter words that begin with a and it doesn't take much time before the computer guesses one of the correct following letters. I've currently set MIN_VALID_WORD_LENGTH = 5 in js/main.js. Perhaps we can give people a slider or something on the webpage somewhere.