feature: support dmg archives for macos

[ekristen]

https://github.com/itchio/butler

https://github.com/itchio/butler/installer/dmg/dmgextract

use case: 99designs/aws-vault

[ekristen]

Unfortunately none of the solutions currently available to me are pretty. I don't want to shell out to hdiutil and there doesn't seem to be a good dmg library in golang.