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*).
```
mkdir "bin"
```

Just renames the onetime executable to 'bin' for me, all the time. If I stick to the current way of mkdir that doesn't happen. The way onetime installs is quite odd, unless I explicitly make and force the onetime executable into a 'bin' folder inside of the ``` /usr/local/cellar/onetime/1.81 ``` folder it installs the executable *outside* of the 1.81 folder, in ``` /usr/local/cellar/onetime ``` and Homebrew doesn't know how to link it in that case.