IO exists!

Ready for review!

Basic framework for the IO subpackage. Still need to add docstrings.
The error messages for sniffer, reader, and writer could be moved into your `DuplicateRegistrationError` class -- this cuts down on duplicate code/messages and might make it easier to test (?). See `skbio.stats.distance.MissingIDError` for an example of how this might be implemented.