Compound formats are dead long live compound formats

Ready for review.

Compound formats have been replaced by the idea of `skbio.io.FileSentinel`. When `FileSentinel` is used as the default for a kwarg, the registry will convert input for that kwarg to an open filehandle.

This is simpler than compound formats.
`skbio.io.read([fastafp, qualfp], format=['fasta', 'qual'])` becomes -> `skbio.io.read(fastafp, format='fasta', qual=qualfp)`
Can you add a comment here explaining why you're attaching type info? i.e., it was previously `FileSentinel = object()`