Consider support for namespaces (nested keys)

[ky1vstar]

trafficstars

I had an idea of implementing namespace feature to separate preferences for different applications modules. I've created extension library here, but what do you think about migrating this feature to the main library.

[sunshinejr]

hey @ky1vstar, this might be really interesting! if there is a way to add that without a breaking change (meaning current users wouldn't need to do much to migrate to a new version) I'd be happy to review it. Thanks for the thought!

[ky1vstar]

@sunshinejr can you review code here https://github.com/ky1vstar/SwiftyUserDefaultsNamespaces and decide wether it can be added to your library? Im afraid I don't have enough time to make PR myself.