Use multiple keys once getKeys becomes available

Related:

- https://github.com/fregante/webext-storage-cache/issues/50

`webext-options-sync` currently stores everything in a single item, this limits the total amount of data a single instance can store.

I can see this after only ~10KB:

By using `getKeys` I could also have a `OptionsSync#get()` method, not just `OptionsSync#getAll()`

It might be a good time to also change serialization method:

- https://github.com/sindresorhus/project-ideas/issues/137