Feature suggestion: Ability to add X.509 certificates as identity proofs

There are some situations when people have verified/trusted third party issued X.509 certificates (e.g. code signing certificates bought from trusted CAs and used for signing WIndows applications).

I suggest adding ability to prove X.509 identity on Keybase by:
1. publishing a third party trusted X.509 certificate (e.g. code signing certificate) on Keybase (without private key, of course; most likely including intermediate certificates)
2. signing Keybase request (containing fingerprints of user's PGP key **and** X.509 certificate) by user's PGP key **and** X.509 certificate
3. publishing both signatures on Keybase as the proof

The issue names no files, tests, or entry points. Start by reviewing Keybase's existing identity-proof flow and determine how trusted X.509 certificates and signatures would fit; done means the proposed certificate publication and dual-signature proof flow is implemented and verifiable.

Written by the indexing model from the issue text.