[Feature Request] Forward specified headers between GRPC backend and web UI

I want to set the login session for an RPC server in a cookie which would require forwarding the Set-Cookie and Cookie headers from the GRPC backend to the web UI.

If I were to make a pull request implementing this, would it likely be accepted?

## Some Implementation options

1. Rename `PreserveHeaders` in [InvokeOptions](https://github.com/fullstorydev/grpcui/blob/master/handlers.go#L50) to `PreserveRequestHeaders` and add a similar `PreserveResponseHeaders` field (which can be set in the same way)
2. Add a `PreserveResponseHeaders` without renaming `PreserveHeaders`
* Leaning for this option since it has minimal effect on existing use cases
4. Apply the existing `PreserveHeaders` to both request and response headers

Start in handlers.go at InvokeOptions and trace how PreserveHeaders is applied to request and response metadata. Compare the listed options with existing header-forwarding behavior; done means the chosen API forwards Cookie and Set-Cookie headers between the gRPC backend and web UI without disrupting existing uses.

Written by the indexing model from the issue text.