Leptus implementation.

Had to add a `url` param when creating a shortened url since leptus is not able to ignore `/` in uri.
Not for the body :frowning:, it does call [cowboy](https://github.com/s1n4/leptus/blob/master/src/leptus_req.erl#L228) but cowboyt itself doesn't have a `body_qs_val/2` function (see [here](https://github.com/ninenines/cowboy/blob/master/src/cowboy_req.erl#L648)), which is weird.