Add support for downloading through a proxy.

[adam-fowler]

[cmcgee1024]

This is already available in the tests, so this might involve moving that functionality into the SwiftlyCore, surfacing it through documented environment variables and command-line options.

This might be a useful bit of research for deciding how the environment variables should work to fit in with a wide variety of other tools and libraries: https://unix.stackexchange.com/questions/212894/whats-the-right-format-for-the-http-proxy-environment-variable-caps-or-no-ca