Add support for xml:base in Atom 1.0

[Vinnl]

Unfortunately RSS 2.0 and JSON Feed do not have an equivalent, but Atom 1.0 allows specifying a base URL that relative URLs inside the feed item's content are relative to. I'm not sure if you're open to features only supported by a subset of the serialisations (although support for JSON Feed's extensions implies so), so feel free to reject.