New API with replicate.run() and OpenAPI-compatible methods

This is a breaking change to the JavaScript client API, but this is still a pre-1.0.0 alpha, so not yet using SemVer.

If you're using previous versions of the client, you'll need to pin to replicate@0.5.0 in you package.json file, or update your code to work with the new client API.

• use new client API (#25) 5a033d0
• add a contributing guide (#23) 6fa9006

https://github.com/replicate/replicate-javascript/compare/v0.5.0...v0.6.0