Field for Library Data in Structs

[aj-foster]

Occasionally, client library authors may want the opportunity to stash additional information in data returned from API requests. I'm considering the addition of a metadata field to all generated structs. This would be similar to the private field in a Plug.Conn struct, or __meta__ in an Ecto schema.

For clarity, a double-underscore field name might be appropriate. It seems prudent to avoid __meta__ in case folks would like to create Ecto schemas out of the generated structs in the future. Perhaps __info__ or __private__ would be appropriate.

Open to thoughts!

cc @feynmanliang and @aej for your thoughts as library implementers.

[aj-foster]

In release 0.0.6, the generator accepts a new option extra_fields that allows library authors to specify one or more additional fields to be added to every schema. This leaves the naming up to the author.

It's likely I will move the configuration option or change how it works in the future, but this is a good first draft.