Default to Object.assign()?

I've been looking at replacing Webpack + Babel with Rollup + Bublé for some small projects I'm working on, and very much resonated with the zero-config approach.

However, one thing that I consistently forget to do is set the `objectAssign` property for Bublé.

Is it possible to have it use Object.assign by default instead of throwing an error?

Start by tracing how Bublé handles the objectAssign option and where the current error is emitted. Review the related behavior and tests, then confirm that omitting the option uses Object.assign by default instead of throwing an error.

Written by the indexing model from the issue text.