Feature request: support trust on first use (TOFU)

[aexaey]

Despite Let's Encrypt's awesomeness, there are still plenty legacy systems around in various intranets that use self-signed certificates.

When one faces a server with self-signed (or expired, or otherwise botched) certificate, and wants to interact with httpie with it, options are:

• --verify yes (default) that provides strong trust guarantee, but usability is lost completely in this particular case;
• --verify no restores usability at cost of trust, which is lost completely.

Middle-ground is TOFU a.k.a. SSH-style certificate verification. OpenSSH client does this. GnuTLS client can do this if --tofu option was specified. Whole thing boils down to:

• On first use, prompt user to manually verify key fingerprint, and stash away a copy;
• On subsequent use, verify server public key against fingerprint learned on first use.

Having --verify tofu as a third option would really nice. Or, without requiring stdio/terminal interaction, one way to implement this would be to print key fingerprint as a part of CERTIFICATE_VERIFY_FAILED error, and verify server's key against fingerprint if one was specified, something like this:

$ http https://old.crusty.server.com/foo
http: error: SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590) while doing GET request to URL: https://old.crusty.server.com/foo
http: info: add "--verify CvuIbn2kasZx0NS6A+Pdzfw1nQ40qtUc6eZXyIH92nM=" to trust this certificate

$ http --verify CvuIbn2kasZx0NS6A+Pdzfw1nQ40qtUc6eZXyIH92nM= https://old.crusty.server.com/foo
HTTP/1.1 200 OK
...