Store feed/entry extra data

[lemon24]

(This is the general version of #276.)

feedparser returns a lot of attributes we're not exposing on Feed/Entry. In the end they should all be exposed, but until we can do so uniformly, it would be nice to give users an escape hatch.

A way of doing this could be an extra: dict[str: Any] attribute, for everything reader can't represent yet.

But... in order for the user to be able to interpret that, we should also add an attribute identifying the parser (Feed.version identifies the feed kind, but doesn't really imply a parser).