help: Cleanup AppHelpTemplate trailing whitespace

Most of the changes here remove trailing whitespace, but I also add
code to select "AUTHOR" or "AUTHORS" as appropriate instead of the
previous "AUTHOR(S)".  The template for listing with an entry per line
is:

   {{range $index, $entry := pipeline}}{{if $index}}
   {{end}}{{$entry}}{{end}}

That range syntax is discussed in [1].

Also add a unit test, which tests both these whitespace changes and
also the earlier App.Description addition.

[1]: https://golang.org/pkg/text/template/#hdr-Variables

app_test.go

@@ -90,7 +90,62 @@ func ExampleApp_Run_subcommand() {
 	// Hello, Jeremy
 }
 
-func ExampleApp_Run_help() {
+func ExampleApp_Run_appHelp() {
+	// set args for examples sake
+	os.Args = []string{"greet", "help"}
+
+	app := NewApp()
+	app.Name = "greet"
+	app.Version = "0.1.0"
+	app.Description = "This is how we describe greet the app"
+	app.Authors = []Author{
+		{Name: "Harrison", Email: "harrison@lolwut.com"},
+		{Name: "Oliver Allen", Email: "oliver@toyshop.com"},
+	}
+	app.Flags = []Flag{
+		StringFlag{Name: "name", Value: "bob", Usage: "a name to say"},
+	}
+	app.Commands = []Command{
+		{
+			Name:        "describeit",
+			Aliases:     []string{"d"},
+			Usage:       "use it to see a description",
+			Description: "This is how we describe describeit the function",
+			Action: func(c *Context) error {
+				fmt.Printf("i like to describe things")
+				return nil
+			},
+		},
+	}
+	app.Run(os.Args)
+	// Output:
+	// NAME:
+	//    greet - A new cli application
+	//
+	// USAGE:
+	//    greet [global options] command [command options] [arguments...]
+	//
+	// VERSION:
+	//    0.1.0
+	//
+	// DESCRIPTION:
+	//    This is how we describe greet the app
+	//
+	// AUTHORS:
+	//    Harrison <harrison@lolwut.com>
+	//    Oliver Allen <oliver@toyshop.com>
+	//
+	// COMMANDS:
+	//      describeit, d  use it to see a description
+	//      help, h        Shows a list of commands or help for one command
+	//
+	// GLOBAL OPTIONS:
+	//    --name value   a name to say (default: "bob")
+	//    --help, -h     show help
+	//    --version, -v  print the version
+}
+
+func ExampleApp_Run_commandHelp() {
 	// set args for examples sake
 	os.Args = []string{"greet", "h", "describeit"}