v4.0.0-rc.0

Use ajv v7
--spec= option to choose JSON schema specification:

• draft7: JSON Schema draft-07 (default)
• draft2019: JSON Schema draft-2019-09

Support schema migration to JSON Schema draft-07 and draft-2019-08 (defined by --spec= option).
Compile multiple schemas to a single module with multiple exported standalone validation functions.
Output standalone validation code to stdout (-o without parameter) for formatting with js-beautify or another utility.