introduce async feature

[softprops]

this crate is blocking back default because, well that's the first usecase I have, but async makes sense if you'd like to depend on that ecosystem and more dependencies

this would mostly be a shift in the trait interface which would likely mean an optional opt in https://docs.rs/async-trait/0.1.51/async_trait/

[mlafeldt]

Hey, I implemented this feature to access DynamoDB from Cloudflare Workers and Deno Deploy:

main...mlafeldt:tiny-dynamo:async

Happy to turn this into a proper PR. As you suggested, this would require an async feature based on async_trait. Wonder if that feature should provide something like get_async and set_async to not break the existing interface?