Need support for authenticated proxy

There's a long thread at #120 talking about the general use of proxies with splinter. The use of unauthenticated proxies is a solved problem, I think, needing only documentation to explain how to setup the profile_preference programmatically (I've created an issue for that #358)

But I don't think there is a solution yet for proxies which require authentication. I've tried all the solutions I've found offered and still haven't found a way I can run splinter against an authenticated proxy, without me having to type the username/password into a dialog box each time the browser opens.

Even when using a stored Firefox profile and setting the 'don't ask for password if we already have it' option.

If this is possible, please put me out of my misery and explain how.

If it would be possible with some coding, please explain the solution and I'll code it.

If it isn't possible even with coding, please explain why.