IIIF Content State URL Parameter

Part of testing and proofing led to the ability to feed objects to the interface for parsing. Right now, I only proofed it for IIIF Manifests. The only available URL pattern is `manifest=`.

From Glen Robson:
"Can you rename the param 'manifest' to 'iiif-content' to hit the IIIF Content state recommendation: https://iiif.io/api/content-state/0.3/"

This proof should support the URL parameter 'iiif-content', hopefully by the time we get to the 2021 IIIF Annual Conference.

Search the interface for the existing `manifest=` URL parameter and trace how it feeds objects into the parser. Consult the IIIF Content State 0.3 recommendation linked in the issue; done means the proof accepts `iiif-content` for IIIF content, with the existing manifest behavior covered or preserved.

Written by the indexing model from the issue text.