Replace URL parser with RegEx with configurable path prefix

[mbklein]

This PR:

• changes the URL parser so that instead of popping path segments off the end of the URL and assuming the image ID is only one segment, it uses a configurable path prefix with a default of /iiif/2/
• increases test coverage
• removes unreachable code paths

This will enable samvera/serverless-iiif#52 and samvera/serverless-iiif#73 to be fixed a lot more easily.

I've included a major version bump because it represents a potentially breaking change to previous parsing behavior.