Update page title on route change

Test Plan:
Navigate to `/`, then click the “Explorer” link, and note that the page
title has updated.

wchargin-branch: update-page-title

src/app/App.js

@@ -3,10 +3,20 @@
 import React from "react";
 import {Router, browserHistory} from "react-router";
 
-import {createRoutes} from "./routes";
+import {createRoutes, resolveTitleFromPath} from "./routes";
 
 export default class App extends React.Component<{}> {
   render() {
-    return <Router history={browserHistory} routes={createRoutes()} />;
+    return (
+      <Router
+        history={browserHistory}
+        routes={createRoutes()}
+        onUpdate={function() {
+          const router = this;
+          const path: string = router.state.location.pathname;
+          document.title = resolveTitleFromPath(path);
+        }}
+      />
+    );
   }
 }

src/app/routes.js

@@ -40,3 +40,19 @@ export function createRoutes() {
     </Route>
   );
 }
+
+function resolveRouteFromPath(path: string): ?RouteDatum {
+  const matches = (candidateRoute) => {
+    const candidatePath = candidateRoute.path;
+    const start = path.substring(0, candidatePath.length);
+    const end = path.substring(candidatePath.length);
+    return start === candidatePath && (end.length === 0 || end === "/");
+  };
+  return routeData.filter(matches)[0] || null;
+}
+
+export function resolveTitleFromPath(path: string): string {
+  const route = resolveRouteFromPath(path);
+  const fallback = "SourceCred";
+  return route ? route.title : fallback;
+}