Ensure discovered performers and spek performers are merged properly

[grosscol]

@id is the JSON-LD specified ID. This is the node IRI in RDF terms.
identifier is the shema.org property https://schema.org/identifier

Cases to handle:

• Performer is listed in the spek with an @id AND identifier e.g. "Alice":

  • The @id listed in the spek is the id used in the output.

• Performer is listed in spek with identifier but no @id e.g. "identifier": "Alice"

  • The blank node id gets assigned as the @id for "Alice" in the output

• Performer is listed in spek with @id but no identifier e.g. "@id": "example.com/Alice"

  • Use given @id iff verbatim @id value is present in the id column of the data
  • Otherwise emit warning that @id not found in identifier column and no identifier specified.

Related to #40