Add the option to use FormData to encode the form elements (#153)

* Add the option to use FormData to encode the form elements

If the form's enctype attribute is set to "multipart/form-data",
use FormData to encode the form's elements.

index.d.ts

@@ -177,6 +177,7 @@ declare namespace Pjax {
       requestUrl?: string;
       requestMethod?: string;
       requestParams?: IRequestParams[];
+      formData?: FormData;
     }
   }