Is it possible to append query values?

I read through the docs but couldn't find any information on how to append query values to my request. In my case, these are conditional, so some methods on the `Builder` struct would be great.

Sadly, simply appending them to the URL is not enough. The values have to be URL-encoded. A utility method similar to other crates would be great. It could take any `T: Serializable` and append the query values. `reqwest` and `ureq` provide this IIRC.

Thanks for the great library!