Enable choosing between storage types.

[mmalenic]

This PR enables choosing between using LocalStorage and AwsS3Storage. It also refactors a bit of code in the some of the crates.

Changes:

• Add StorageType config option which allows choosing between LocalStorage and AwsS3Storage.
• Refactor some config options.
• Update usage text.
• Add a few tests for the config options.

Further improvements may involve adding the ability to setup multiple resolvers, that map to arbitrary storage types. For example, an id starting with "aws-{id}" could map to an AwsS3Storage, whereas an id starting with "local-{id}" could map to LocalStorage.

[brainstorm]

Other than the failing tests, thanks for this @mmalenic! ;P

Now we can finally change between storage backends along with the massive config refactor, awesome. Let's tackle lambdas and API Gateways next, shall we? :)