Luhn checksum calculator for the terminal.
$ luhn 7992739871 3
Download the binary into your bin folder, or
build and then copy from .build/release/luhn into your bin folder (
cp .build/release/luhn /usr/local/bin/luhn on OS X).
clone the repo and run
swift build --configuration release in the project directory. See [https://swift.org] if you don't have swift installed.