Add SSSOM and RDF outputs

[cthoyt]

This PR adds two new output formats:

Format	File	Description
SSSOM	tiwid.sssom.tsv	An output in the Simple Standard for Sharing Ontological Mappings (SSSOM) format
RDF	tiwid.ttl	An output in RDF, serialized with Turtle. This file can be readily converted to an OWL ontology with ROBOT with robot convert -i tiwid.ttl -o tiwid.owl.

It also makes minor updates to the README, such as noting that the data is standardized on Bioregistry and not Identifiers.org (though, effectively this is the same thing)

[egonw]

Thanks, great idea!