Added ngModelElAttrs property

The field configuration object supports a property called `ngModelElAttrs` which allows you to place attributes with string values on the ng-model element. It's a straightforward alternative to ngModelAttrs and was implemented as a result of this issue: https://github.com/formly-js/angular-formly/issues/378#issuecomment-127211353

angular-formly/angular-formly.d.ts

@@ -301,6 +301,17 @@ declare module AngularFormly {
 		};
 
 
+		/**
+		 * This allows you to place attributes with string values on the ng-model element.
+		 * Easy to use alternative to ngModelAttrs option.
+		 *
+		 * see http://docs.angular-formly.com/docs/field-configuration-object#ngmodelelattrs-object
+		 */
+		ngModelElAttrs?: {
+			[key: string]: string;
+		};
+
+
 		/**
 		 * Used to tell angular-formly to not attempt to add the formControl property to your object. This is useful
 		 * for things like validation, but not necessary if your "field" doesn't use ng-model (if it's just a horizontal
@@ -572,4 +583,4 @@ declare module AngularFormly {
 		messages: { [key: string]: ($viewValue: any, $modelValue: any, scope: ITemplateScope) => string };
 	}
 
-}
+}