Add "idem" for identical author and editor

[PinkSerenity]

Some citation styles (like the one my university uses) require the usage of id. to refer to an editor who has already been mentioned in the same citation as an author.

Example:

Doe, John: "A chapter". In: id. (ed.): A book. Place: Publisher Year, p. 1-99

I do not think that this is a good decision, but that doesn't matter, since it's a requirement and not my responsibility to argue against that.