Add virtual features to Dictionary

This is the _bare_ minimum change to convert `FeatureMeta` into an ADT (except for the dense row change).

Now is a Good Time to get the naming/layout of this new constructor correct.

The next step will be to add support for the dictionary parsing, and then thrift support.

/cc @etorreborre @raronson @markhibberd 
> Why is a dummy FeatureMeta being returned here?

Good question. We want to write out a column for the virtual feature when we eventually add it. The only question will be what encoding it is, based on the query. I might replace this with a `NotImplemented` for now, just to avoid any confusion.