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*).
Will do. I couldn't find any official checksums, so I ended up calculating my own and went with the theory that security software is generally distributed with sha256 rather than sha1. Happy to use either though and will drop it back to sha1 in the next commit.