Support proxy exception list

[dcoutts]

I'm glad to see the new auto-proxy support, much more convenient!

A closely related feature is that more complicated proxy configurations have exception lists, e.g. no_proxy=localhost, listing servers (usually on the local LAN) that should not go via the proxy. Windows allows this, and it seems from a quick search that on unix the no_proxy env var can be used. At least, wget and w3m use it.

I know of a user who actually needs a setup like this, everything goes through a proxy, but they have some servers on localhost which the proxy of course is not aware of.

So I think what'd we'd need is to extend the Proxy constructor with such a list, and then check when we're about to make a connection if the target host is on the list and if so to make a direct connection rather than using the proxy.