'allowed' value addition for subject_is_issuer

[csuwildcat]

@brentzundel sorry for all those changed lines, it's all just linting formatting the file, the only actual addition is the allowed value for subject_is_issuer, and the note that talks about its use cases.

[csuwildcat]

@brentzundel on the call we discussed the allowed property and folks were wondering about the preferred value itself perhaps being replaced by allowed for all the properties that use it. I was only looking for the additional property, but the logic presented seemed to make sense. However, you knew that part of the spec best, so we want to have your input on what preferred does vs something like allowed, and get your recommendation on what to do.