Consider adding kgcl bindings

[cmungall]

KGCL is a proposed standard for representing changes to ontologies. See:

https://github.com/INCATools/kgcl

There are currently bindings for this and rdflib:

https://github.com/INCATools/kgcl-rdflib

It would be great to have pronto bindings - meaning people could just send a KGCL file to an obo file and have it update

Understandable if you don't want the dependency in this repo; it brings in rdflib and others...

options:

1. kgcl bindings live in this repo
2. there is a separate kgcl-pronto lib, analogous to the rdflib one
3. the bindings like in OAK, which acts as a big uber wrapper

I am going with 3 for now but happy to have them anywhere