Add open source license

[xdg]

Hi, my subway-obsessed 8-year-old absolutely loves playing with this game. So thank you for writing it!

I told him I'd see if the author would open up the source so I could look into minor issues/features and, 'lo, it was already on GitHub. 😄

Since you're already publishing this on GitHub, would you mind also slapping an open source license on it? I see you used the MIT license for foobot-slack; that would be a fine choice for this sort of fun project.

[pinecone460]

Any update on this? I know that enmodal, which seems to be based on the same code, is open source, but it currently lacks a public implementation, as well as, at least from the local implementation I tried, JSON import. It would be great if this were available under an open source license -- currently I can't seem to find any software of the sort that is.

[bootmii]

@pinecone460, what is that license?