support generate_appcast

[fanchou]

I use CI pipeline to automatically release the version, so I need to be able to automatically update appcast.xml. Hope to support generate_ appcast.

[cbenhagen]

The binary can be called directly from ./macos/Pods/Sparkle/bin/generate_appcast. That's what we use in our pipelines. If that's not enough, please provide additional details of what you would expect from support in this package.