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*).
`system "#{bin}/onetime", "--version"` in Ruby runs the equivalent of `/usr/local/Cellar/onetime/1.2.3/bin/onetime --version` in Bash.