text/template: add example showing use of custom function

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5694100

src/pkg/text/template/examplefunc_test.go

+// Copyright 2012 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 template_test
+
+import (
+	"log"
+	"os"
+	"strings"
+	"text/template"
+)
+
+// This example demonstrates a custom function to process template text.
+// It installs the strings.Title function and uses it to
+// Make Title Text Look Good In Our Template's Output.
+func ExampleTemplate_func() {
+	// First we create a FuncMap with which to register the function.
+	funcMap := template.FuncMap{
+		// The name "title" is what the function will be called in the template text.
+		"title": strings.Title,
+	}
+
+	// A simple template definition to test our function.
+	// We print the input text several ways:
+	// - the original
+	// - title-cased
+	// - title-cased and then printed with %q
+	// - printed with %q and then title-cased.
+	const templateText = `
+Input: {{printf "%q" .}}
+Output 0: {{title .}}
+Output 1: {{title . | printf "%q"}}
+Output 2: {{printf "%q" . | title}}
+`
+
+	// Create a template, add the function map, and parse the text.
+	tmpl, err := template.New("titleTest").Funcs(funcMap).Parse(templateText)
+	if err != nil {
+		log.Fatalf("parsing: %s", err)
+	}
+
+	// Run the template to verify the output.
+	err = tmpl.Execute(os.Stdout, "the go programming language")
+	if err != nil {
+		log.Fatalf("execution: %s", err)
+	}
+
+	// Output:
+	// Input: "the go programming language"
+	// Output 0: The Go Programming Language
+	// Output 1: "The Go Programming Language"
+	// Output 2: "The Go Programming Language"
+}