Security issue: UNIX sockets are always allowed (opt-in required)

In the documentation here: https://visionmedia.github.io/superagent/#request-basics is written that

> The Node client supports making requests to Unix Domain Sockets

Such as `pattern: https?+unix://SOCKET_PATH/REQUEST_PATH`. This seems like a major security concern and should probably be disabled by default. Can we make an option like `.enableUnixSockets()` on superagent and disable it by default?

I'm not too familiar with UNIX sockets but I suppose something like matching `unix://` with a regex will do. Then by default rejecting / throwing an error for all requests that contain it unless `.enableUnixSockets()` is enabled by the developer.

If the URL input is dynamic and possibly untrusted it is very much disagreeable to allow unix sockets there.

Start with the request-basics documentation at https://visionmedia.github.io/superagent/#request-basics and trace the Node client path that handles the documented https?+unix:// pattern. Determine where Unix socket URLs are accepted, then verify that they are rejected by default and allowed only through an explicit opt-in, with coverage for both behaviors.

Written by the indexing model from the issue text.