Add greedy policy to Chain class

[AlliedToasters]

I was using markovify for a non-text use case and found the greedy policy helpful, so I added this method. Feel free to add, no worries if not!

[jsvine]

Hi @AlliedToasters, and thanks for sharing this. It seems like an interesting, well-scoped addition. Would you be open to adding test coverage for it? If not, no worries — I can try doing that myself, although I can't promise I'll get to it soon.