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*).
If I manually call the test:
```
brew test /Users/Bob/Downloads/onetime.rb
Testing onetime
==> /usr/local/Cellar/onetime/1.81/bin/onetime --version
```