Hello!

To start things off, awesome job with this library, it does what it sets out to do very well.

I have currently 2 apps that I'd like to share the UUID with, but I'm struggling a bit understanding the shared keychain access. Usually when sharing keychains we end up with 2 keychains. Example:

bundleId.app.one, bundleId.app.shared
bundleId.app.two, bundleId.app.shared

This allows us to only share the shared keychain while keeping everything else in the private keychain. I don't see where we specify the keychain for FCUUID to use, though, so how does it work exactly? I imagined we'd need to do something like:

FCUUID.defaultAccessGroup = "bundleId.app.shared"

Any pointers on this? If this is just an unsupported feature I'm fine creating a PR with the implementation if needed

Thanks!