Add prefetch to propTypes + flow typings (#1295)

Author: timneutkens

examples/with-flow/types/next.js.flow

@@ -17,7 +17,7 @@ declare module "next/head" {
 }
 
 declare module "next/link" {
-  declare module.exports: Class<React$Component<void, {href: string}, any>>;
+  declare module.exports: Class<React$Component<void, {href: string, prefetch?: bool}, any>>;
 }
 
 declare module "next/error" {

lib/link.js

@@ -10,6 +10,7 @@ export default class Link extends Component {
   }
 
   static propTypes = {
+    prefetch: PropTypes.bool,
     children: PropTypes.oneOfType([
       PropTypes.element,
       (props, propName) => {