This addresses #11255. A request to implement support for status
codes other that 301/302 for this plugin, as Netlify also supports
other status codes.

@sidharthachatterjee suggested I submitted a PR
with my proposed changes, and this is that PR.

It adds one option to the `createRedirect` call which is `statusCode`.
You can use it to manually set the status code. If it's not set, the
status code will be determined by the `isPermanent` option as before
(`301` if `true` or `302` if not (the default) ).

I've also updated the README to document this new feature.

Co-Authored-By: joostdecock <joost@decock.org>