path/filepath: add example for Ext

Make it dead simple to see visually what the function outputs in
various scenarios.

Change-Id: I8f6fcd72fa1515361481f0510412cde221e1d4e3
Reviewed-on: https://go-review.googlesource.com/51630
Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Hiroshi Ioka <hirochachacha@gmail.com>

src/path/filepath/example_test.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package filepath_test
+
+import (
+	"fmt"
+	"path/filepath"
+)
+
+func ExampleExt() {
+	fmt.Printf("No dots: %q\n", filepath.Ext("index"))
+	fmt.Printf("One dot: %q\n", filepath.Ext("index.js"))
+	fmt.Printf("Two dots: %q\n", filepath.Ext("main.test.js"))
+	// Output:
+	// No dots: ""
+	// One dot: ".js"
+	// Two dots: ".js"
+}