Extend x509 grammar

Support XMPP certificates as per [RFC6120](http://tools.ietf.org/html/rfc6120#section-13.7.1.4): recognize XMPP OID as part of `GeneralName.OtherName` in `Subject Alternative Name`, `Issuer Alternative Name` and `Authority Key ID` extensions.

Fixes the cert used by [CCC](https://ccc.de).