Fix definition of ReactModal.setAppElement

This comit changes:
- definition of static setAppElement method to support string type - as
ReactModal allows to use selector instead of element reference
- sample tests to include static method coverage

Thanks!

types/react-modal/index.d.ts

@@ -4,7 +4,8 @@
 //                 Drew Noakes <https://github.com/drewnoakes>,
 //                 Thomas B Homburg <https://github.com/homburg>,
 //                 Tatu Tamminen <https://github.com/ttamminen>,
-//                 Uwe Wiemer <https://github.com/hallowatcher>
+//                 Uwe Wiemer <https://github.com/hallowatcher>,
+//                 Peter Blazejewicz <https://github.com/peterblazejewicz>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -97,6 +98,9 @@ declare namespace ReactModal {
 declare class ReactModal extends React.Component<ReactModal.Props> {
     /* Override base styles for all instances of this component. */
     static defaultStyles: ReactModal.Styles;
-    /* Call this to properly hide your application from assistive screenreaders and other assistive technologies while the modal is open. */
-    static setAppElement(appElement: HTMLElement): void;
+    /**
+     * Call this to properly hide your application from assistive screenreaders
+     * and other assistive technologies while the modal is open.
+     */
+    static setAppElement(appElement: string | HTMLElement): void;
 }

types/react-modal/react-modal-tests.tsx

@@ -1,6 +1,12 @@
 import * as React from "react";
 import ReactModal = require('react-modal');
 
+// tests for static method
+// string
+ReactModal.setAppElement("#main");
+// HTMLElement
+ReactModal.setAppElement(document.getElementById("#main"));
+
 class ExampleOfUsingReactModal extends React.Component {
   render() {
     const onAfterOpenFn = () => { };