WIP: getKeyEnv, replace file storage with environment variable

[Reference](https://github.com/yesodweb/yesod/issues/806).

The basics of the idea are here, but it's not quite ready.

TODO:

- [ ] I'm open to better names for the functions introduced and the executable
  I added. These seem good to me, but they aren't what was discussed in the
  reference PR.

- [ ] The Base64-encoded string which gets generated, printed, and stored in
  the environment variable still has some funny characters that make my
  terminal go wonky, I'm assuming this means it's incorrect -- I'm a bit stuck
  on where I went wrong or what I need to do.

Would love some early feedback / pointers.

Thanks!