CBImport should have the ability to omit certain fields

Summary: When importing data from CSV, it would be helpful to be able to omit certain fields from being loaded into the document. A practical application would be that you want to use a certain field to construct the document key, but don't want to include that field in the actual content.

Suggestion: Maybe another flag like --omit-fields fieldname1,fieldname2, etc? Any fields being omitted should still be able to be used with --generate-key