Support async requests

I recently found this crate and absolutely love it. Thanks for the amazing project and keep up the good work! :+1:

During usage I wonder why the provided functions aren't `async` as the data must be fetched from somewhere. After looking into the code base I found that we currently use blocking requests.

At first it seems like an easy implementation for me, since we just have to migrate the usage of the blocking `reqwest` client to a non blocking and transform some traits and functions to `async`. What maybe could be implemented with a optional feature flag. However, after trying to prototype this on my own fork (https://github.com/jontze/geocoding/tree/feat/async-fetching) I came to the conclusion that it is not that easy :laughing: . Since it looks like that there have to be done some refactor of at least the `Point` struct in the [geo-types](https://github.com/georust/geo/tree/master/geo-types) crate to support rust futures.

Furthermore, a feature flag is definitely not the cleanest solution to implement this since it will lead to some code duplication, I guess. On the other side transforming the whole crate to async will be a major breaking change for the user...

However, before continuing on that, are there any plans for the future to support async data fetching instead of blocking?