Proxy configuration

[wojtekmach]

We should either document how to set up proxy (https://github.com/hexpm/hex_core/pull/36#issuecomment-415528275) or have first-class support for setting it (either there's a contract for it or adapter-specific configuration is passed down so we can e.g. httpc:set_options() in the httpc adapter)

Ref: https://github.com/wojtekmach/mini_repo/issues/8