add placholder parameter to work with codemirror placeholder addon

codemirror/codemirror.d.ts

@@ -787,7 +787,10 @@ declare module CodeMirror {
         viewportMargin?: number;
 
         /** Optional lint configuration to be used in conjunction with CodeMirror's linter addon. */
-        lint?: boolean | LintOptions;
+        lint?: boolean | LintOptions;
+
+	/** Optional value to be used in conduction with CodeMirror’s placeholder add-on. */
+	placeholder?: string;
     }
 
     interface TextMarkerOptions {