[Twisted-web] IPolicyForHTTP supporting cert based mutual auth

[Jason Litzinger]

Recently, I had to implement a client that supported cert based mutual
auth.  I searched a bit in both Twisted and treq but didn't find an
existing implementation (maybe I missed it), so I implemented a custom
IPolicyForHTTPS.  It was pretty easy, and though it needs more testing,
seems to be working well.

My question is whether this would be of interest as a PR to Twisted
itself (or a project under that umbrella)?  The use case comes to mind is IoT
(though it isn't the only one), and a specific example is the AWS IoT API:
http://docs.aws.amazon.com/iot/latest/developerguide/authorization.html

Cheers,
-Jason Litzinger