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

help.go

@@ -16,27 +16,28 @@ var AppHelpTemplate = `NAME:
    {{.Name}} - {{.Usage}}
 
 USAGE:
-   {{if .UsageText}}{{.UsageText}}{{else}}{{.HelpName}} {{if .VisibleFlags}}[global options]{{end}}{{if .Commands}} command [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}{{end}}
-   {{if .Version}}{{if not .HideVersion}}
+   {{if .UsageText}}{{.UsageText}}{{else}}{{.HelpName}} {{if .VisibleFlags}}[global options]{{end}}{{if .Commands}} command [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}{{end}}{{if .Version}}{{if not .HideVersion}}
+
 VERSION:
-   {{.Version}}
-   {{end}}{{end}}{{if .Description}}
+   {{.Version}}{{end}}{{end}}{{if .Description}}
+
 DESCRIPTION:
-   {{.Description}}
-{{end}}{{if len .Authors}}
-AUTHOR(S):
-   {{range .Authors}}{{.}}{{end}}
-   {{end}}{{if .VisibleCommands}}
+   {{.Description}}{{end}}{{if len .Authors}}
+
+AUTHOR{{with $length := len .Authors}}{{if ne 1 $length}}S{{end}}{{end}}:
+   {{range $index, $author := .Authors}}{{if $index}}
+   {{end}}{{$author}}{{end}}{{end}}{{if .VisibleCommands}}
+
 COMMANDS:{{range .VisibleCategories}}{{if .Name}}
    {{.Name}}:{{end}}{{range .VisibleCommands}}
-     {{join .Names ", "}}{{"\t"}}{{.Usage}}{{end}}
-{{end}}{{end}}{{if .VisibleFlags}}
+     {{join .Names ", "}}{{"\t"}}{{.Usage}}{{end}}{{end}}{{end}}{{if .VisibleFlags}}
+
 GLOBAL OPTIONS:
-   {{range .VisibleFlags}}{{.}}
-   {{end}}{{end}}{{if .Copyright}}
+   {{range $index, $option := .VisibleFlags}}{{if $index}}
+   {{end}}{{$option}}{{end}}{{end}}{{if .Copyright}}
+
 COPYRIGHT:
-   {{.Copyright}}
-   {{end}}
+   {{.Copyright}}{{end}}
 `
 
 // CommandHelpTemplate is the text template for the command help topic.