Support for DMG files (macOS)

[devinstasafe]

It is not mystery that each release of macOS has increased the default level of security. The latest one won't even run any binaries that is not notarized.

Since notarization only works with .dmg files, it'll be great if go-selfupdate supports this format just like electron builder updates.

Note :: A binary can be signed as .zip file or notarized + stapled as .dmg

Although .dmg is ideal, support for .zip should also do that trick.

[sanbornm]

I'm not sure how to do this. If you could link to documentation on this it would be helpful.