pkgbuild

Consider integrating `pkgbuild` into the build system for creating MacOS install packages.

```shell
APP_NAME=ttcards VERSION=0.8.0 pkgbuild \
  --root ~/Projects/ttcards.git/build/Debug/${APP_NAME}.app \
  --version ${VERSION} \
  --identifier com.i8degrees.${APP_NAME} \
  ./${APP_NAME}_${VERSION}.pkg
```