Add success flag property

JSONSchema validation results have a `valid` bool that certifies whether the validated JSON came back positive or not. When importing these definitions into my project, this property wasn't there, so I had to add it locally. It should be there.

jsonschema/index.d.ts

@@ -1,6 +1,6 @@
 // Type definitions for jsonschema
 // Project: https://github.com/tdegrunt/jsonschema
-// Definitions by: Vlado Tešanovic <https://github.com/vladotesanovic>
+// Definitions by: Vlado Tešanovic <https://github.com/vladotesanovic>, kinesivan <https://github.com/kinesivan>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 declare module "jsonschema" {
 
@@ -11,6 +11,7 @@ declare module "jsonschema" {
         propertyPath: string;
         name: string;
         schema: {};
+        valid: boolean;
         throwError: any;
         disableFormat: boolean;
     }