Use new Chronicle Schema work from chronicle-core

[hyfen]

Chronicle Schema is an ontology for defining valid classes and their properties/edges for representing digital personal history. It's a derivate (but not strict-subset) of schema.org's ontology.

This PR brings Chronicle-ETL into line with the schema work that got added to https://github.com/chronicle-app/chronicle-core.

This PR also adds:

• ability to have multiple transformers. Closes Jobs should be able to have multiple transformers #6
• a convention for plugin strategies. Closes Plugin connectors should be able to specify different strategies #56
• ability to load a Ruby connector file from filesystem. Closes Users should be able to load arbitrary connector classes #23