Support building a Downloader using a provided (and pre-configured) reqwest::Client

[dralley]

A user might want to use reqwest::Client features which this library doesn't have support for configuring - one easy and generic way around that would be to allow building a downloader with a provided request::Client, skipping the normal configuration steps in that case.

e.g.

• proxy configuration
• tls client cert configuration
• custom CA bundles

It might also make sense to support those features directly and I can file separate issues for those if you agree.

[hunger]

Sounds like a good addition, but I will most likely not get round to implement any of them anytime soon:-( But contributions are welcome!