Customise shim per entrypoint

Hi! I have been using this package for a little while, and stumbled now upon a slight issue. When creating a multi-entrypoint package where each entrypoint supports a different subset of features, for example:

- one entrypoint for system, where fs and env is needed
- one entrypoint for browser, where document is needed
- one entrypoint for react components etc

It would be great if each entrypoint (the interface `EntryPoint`) would contain a `shims` property (interface `ShimOptions`) that would override the top-level `shims` property.