Async variant request

[michalfita]

Thanks for your crate and efforts to maintain it - it's very pragmatic.

But as world moves forward the need for Async Rust grow, hence my kind request to consider adding asynchronous implementation. There's a fork which attempted to do so, but since then doesn't seem actively maintained. It could be used as inspiration.

If I found time I'll produce a PR myself, but nice integration doesn't look like job for single PR - common components would need to be extracted first for maximum reuse. Simple copy&paste of lib.rs is asking for maintenance hell in future.

Thank you for consideration.