Allow Go code comments to be used as source of schema descriptions

[samlown]

This is a bit of an experimental feature for consideration.

Our use-case is that we want to be able to use a single set of "descriptions" for both Go documentation and the generated JSON Schema. It get's boring very quickly adding both a comment describing a type of field, and a jsonschema_description.

This PR adds a CommentsMap property to the reflector which is essentially a dictionary that maps fully qualified package paths alongside types and fields, pointing to comment strings.

The AddGoComments method included in the Reflector takes a base import module and a path, extracts all the comments, and adds them to the CommentsMap:

r := new(jsonschema.Reflector)
if err := r.AddGoComments("github.com/alecthomas/jsonschema", "./examples"); err != nil {
  // deal with error
}

I couldn't figure out a way to automatically determine the full module name, so it has to be defined manually for each package you'd like to use comments from.

The end result is that you can define Go comments, and have them imported into your Schema. The tests provide a complete example of the results. One of the big advantages of this approach is that it makes it much easier to add descriptions to types.

[alecthomas]

This is an interesting idea, I quite like it.

[alecthomas]

I think it would be good to move this to a new file, this is getting quite long.

[samlown]

Agreed. I'll see what I can do.

[samlown]

I've extracted the main comment parser part to it's own file with a "reflector independent" method. Please let me know if you'd prefer a different approach!

[alecthomas]

Thanks!