Make the storage backend configurable

[fsmanuel]

The discussion about suporting more storage backends has been sitting there for a while now #153, #255

The main blocker is that it requires an async api to the storage as both localForage and chrome.storage.local have an async api.

To not have breaking changes we would need to support both sync and async apis and a clear path to let the users migrate to the async api.

• Implement async version
• Document usage of sync and async api

Join the discussion: #338

Refs:

• Discussion: Local Forage, moving beyond LS #153 (comment)
• chrome.storage.local #255 (comment)
• Feat(localForage): Add localForage support #155
• Local Forage support #278