Support dangling classes

[cmungall]

I'm splitting this off from

• Is there a way to adjust the obo parsing strictness? #159

As I think this is a distinct case. There is an argument for pronto remaining strict wrt the spec, but the spec doesn't forbid dangling classes. The spec defines a profile called OBO Basic, that forbids dangling classes, https://owlcollab.github.io/oboformat/doc/obo-syntax.html#6.2

However, an ontology can be valid without conforming to OBO Basic

We want to push out Base modules, where dangling classes are the norm, it would be great if pronto/fastobo handles these.

Originally posted by @cmungall in #159 (comment)