Single player works (mostly)

Using proper user inputs, you can now run through a text-based solitaire
version of yahtzee without the yahtzee bonus and get your final score.
I'll add the yahtzee bonus asap, then work on making user input more
flexible. If you type keep_dice in wrong format, it returns an error,
and I've probably missed a couple other bugs. Since I am planning on
creating a simple GUI for this project, I will not be catering too much
to text input flexibility, but I would still like to see it work without
errors. Also up next will be implementing multiple players. This may
take some refactoring, but shouldn't be too bad.