Add support for using prefix.cc for resolving new prefixes to namespace URIs

[hlapp]

At present the user has to provide the namespace URI for prefixes used in metadata attributes. But for well-known prefixes, the package could leverage the crowd-sourced resource prefix.cc. (I've also just registered the prefix nex there).

[cboettig]

nice, that's a great idea. presumably this might be used if no prefix was supplied, and otherwise supplying a prefix would allow a user to override the default?