Add a new processing spec document

[bdarcus]

This would add a suggested API, based on existing CSL implementations.

Beyond basics of citation processing, it would include the features in #71 and #76, in part to make clear this is out of scope for CSL styles per se.

[bdarcus]

I broadened this based on citation-style-language/test-suite#13.

I think we need a separate document aimed explicitly at implementers, that fills in missing expectations on processing, suggested API, etc.