Add `.dmg` installer option for macOS

[timolins]

When I built my first Photoshop extension, I also had to to find a way to make it easy to install. This package wasn't around back then, so I played around with some stuff and ended up creating a script that creates custom .dmg file. It includes the extension itself as well as a symbolic link to the Photoshop extensions directory.

I'm not sure if this should be added to this package, but it worked pretty well for me so far, so I thought I'd share it.

A pretty neat side-effect of this is that you don't need to sign the .dmg, since the user never actually runs an executable.

My currently workflow is to create a .zxp with create-zxp and then use it to create the .dmg as well as the Windows installer.

Let me know what you think!

[vespakoen]

Hey! Thats a pretty sweet implementation indeed and basically what the exe and pkg are doing right now. I think I will not add support for it to this package though.

Feel free to make a PR for it though i'd happily include it.