Extract proxy basic auth from URL

[Coffei]

Since this library can automatically use ENV variables to set proxy, it would be cool if it could also set proxy basic auth, if it's present in the URL. We have a proxy ENV variable defined as HTTPS_PROXY=http://user:password@proxy.example.com:80/. I checked the code and it seems while proxy is configured from both explicit configuration options and ENV, proxy auth is only configured via explicit config options.

[benoitc]

I will add this support in #737 . Thanks for the suggestion :)