Add support for specifying JSON schema dialects (fixes #202)

Sources/OpenAPIKit/OpenAPI.swift

@@ -1,9 +1,60 @@
 //
 //  OpenAPI.swift
-//  
+//
 //
 //  Created by Mathew Polzin on 6/22/19.
 //
 
 /// The OpenAPI namespace
 public enum OpenAPI {}
+extension OpenAPI {
+    public struct Document: Equatable {
+        public var openAPIVersion: Version
+        public var info: Info
+        public var servers: [Server]?
+        public var paths: PathItem.Map
+        public var webhooks: OrderedDictionary<String, Either<JSONReference<PathItem>, PathItem>>?
+        public var components: Components?
+        public var security: [SecurityRequirement]?
+        public var tags: [Tag]?
+        public var externalDocs: ExternalDocumentation?
+        public var jsonSchemaDialect: URL?
+
+        public init(
+            openAPIVersion: Version = .v3_1_0,
+            info: Info,
+            servers: [Server]? = nil,
+            paths: PathItem.Map,
+            webhooks: OrderedDictionary<String, Either<JSONReference<PathItem>, PathItem>>? = nil,
+            components: Components? = nil,
+            security: [SecurityRequirement]? = nil,
+            tags: [Tag]? = nil,
+            externalDocs: ExternalDocumentation? = nil,
+            jsonSchemaDialect: URL? = nil
+        ) {
+            self.openAPIVersion = openAPIVersion
+            self.info = info
+            self.servers = servers
+            self.paths = paths
+            self.webhooks = webhooks
+            self.components = components
+            self.security = security
+            self.tags = tags
+            self.externalDocs = externalDocs
+            self.jsonSchemaDialect = jsonSchemaDialect
+        }
+
+        private enum CodingKeys: String, CodingKey {
+            case openAPIVersion = "openapi"
+            case info
+            case servers
+            case paths
+            case webhooks
+            case components
+            case security
+            case tags
+            case externalDocs
+            case jsonSchemaDialect = "$schema"
+        }
+    }
+}