new formula: onetime

New formula: onetime. Assists users in creating one-time pads. Still actively supported upstream, and has gathered a lot of renewed interest thanks to Hak5 covering the tool recently.

(*This is my first formula created entirely from scratch, and I'm pretty thrilled to have actually managed to make it work around Homebrew's syntax & formula requirements. Extremely thoroughly tested, so hopefully will pass the build bot first time*).
Can a test be added to do something more substantial then e.g. `--version` or `--help`? See `cmake.rb` for an example of a formula with a good test. Thanks!