Proposal to enable customizing of resolvergen through additional plugins

I would like to auto-generate code in the resolvers, where we usually have `panic(not implemented)`, by passing the `Field` object to a plugin at this point during codegen. [Here](https://github.com/shrdlu68/gqlgen/commit/f8c83a9291566141045b45c84c4084009250d4a3) is an example of what this would look like.