meta should be able to handle arbitrary fields

[dustmop]

Issue #26 leaves out an important detail: the meta component should be able to hold any arbitrary field value. Any field name not in its "known" list should be put into a generic map of string->string. See the declaration of qri's Meta struct at https://github.com/qri-io/dataset/blob/master/meta.go which has a private meta field for this purpose. Note how this is handled using the fill package which looks for an ArbitrarySetter interface, which is defined alongside the Meta struct.

This python package does not support this behavior, but it should, and needs accompanying tests.