owner.go

As highlighted in #48, it would be handy to have a means for filtering
the list of repositories by the owner.

We can't be too strict on what `owner` means in this case, so we can
accept free-form text, as well as a `notes` for supporting information.

This wires it in to be usable, including adding a domain model, but is
not yet used anywhere.

Closes #48.