Custom UserDefaults suite

[Maschina]

This PR adds the capability to define a custom UserDefaults to support App Groups and access group containers.

I am using KeyboardShortcuts in an environment with App Groups. The shared UserDefaults shall also include the defined shortcuts.

[orchetect]

Being able to substitute UserDefaults suites is valuable in other respects. In an app where I use the package, for unit tests and integration tests we use a custom suite for prefs, and it would be useful to be able to use the same suite with KeyboardShortcuts.

[Maschina]

I am just realizing that event streams also require observations. Will make another commit for that.

[sindresorhus]

Make sure you manually test it in the example app and also a real app to ensure it doesn't introduce any new bugs. I'm a little bit nervous about large changes like this as we have no real tests.