Handle all top-level objects in create_metadata?

[Robsteranium]

The spec permits a few top level objects: table group, table, schema, dialect or transformation.

It would be good if create_metadata and read_csvw could support tables and not just table groups, perhaps switching depending on whether a property named "url" vs "tables" is present.

It might also make sense to handle the other types too.

This puts more responsibility on the user to construct lists that comply with the spec (rather than having csvwr take responsibility for this).