Top-level objects do not support additionalProperties

[antoineco]

Issue description

additionalProperties is only supported for nested schemas.

example

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "$id": "http://example.com/parameters.jschema",
    "title": "parameters",
    "type": "object",
    "properties": {
        "field": {
            "type": "object",
	    "additionalProperties": { "type": "string" }
        }
    }
}

generates

package main

// Parameters 
type Parameters struct {
    Field map[string]string `json:"field,omitempty"`
}

I would expect top-level schemas to also support additionalProperties.

example

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "$id": "http://example.com/parameters.jschema",
    "title": "parameters",
    "type": "object",
    "additionalProperties": { "type": "string" }
}

generates

package main
                   /*                          expected */
// Root            /*    // Field                       */
type Root string   /*    type Field map[string]string   */

[antoineco]

This should be a simple addition to #40, I'm working on it.