getInstance with only string not working

According to the docs https://db-migrate.readthedocs.io/en/latest/API/programable/ this should work: `let dbmigrate = DBMigrate.getInstance(true, './path/to/database.json');`

Probably because here https://github.com/db-migrate/node-db-migrate/blob/master/api.js#L104 never the type string is checked. Should I do a PR?

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