CLI: no way to replace default array values

[isker]

The default arrays like https://github.com/reactjs/react-docgen/blob/e05219b08c8b17899be812a521b4985f2b66fae2/bin/react-docgen.js#L34 can only be appended to, not replaced, by passing values on the CLI. This is most consequential for the one that I linked, as it's not possible to exclude '.js' files from processing except by adding an --exclude regex that overrides the default.