Properly detect Zsh shell

There is no need to define custom shell var, when Zsh can be detected by
checking SHELL env var.

app_test.go

@@ -355,7 +355,7 @@ func ExampleApp_Run_bashComplete() {
 func ExampleApp_Run_zshComplete() {
 	// set args for examples sake
 	os.Args = []string{"greet", "--generate-bash-completion"}
-	_ = os.Setenv("_CLI_ZSH_AUTOCOMPLETE_HACK", "1")
+	_ = os.Setenv("SHELL", "/usr/bin/zsh")
 
 	app := NewApp()
 	app.Name = "greet"

autocomplete/zsh_autocomplete

@@ -4,9 +4,9 @@ local -a opts
 local cur
 cur=${words[-1]}
 if [[ "$cur" == "-"* ]]; then
-  opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} ${cur} --generate-bash-completion)}")
+  opts=("${(@f)$(${words[@]:0:#words[@]-1} ${cur} --generate-bash-completion)}")
 else
-  opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} --generate-bash-completion)}")
+  opts=("${(@f)$(${words[@]:0:#words[@]-1} --generate-bash-completion)}")
 fi
 
 if [[ "${opts[1]}" != "" ]]; then

docs/v2/manual.md

@@ -1211,14 +1211,13 @@ func main() {
 
 #### ZSH Support
 Auto-completion for ZSH is also supported using the `autocomplete/zsh_autocomplete` 
-file included in this repo. Two environment variables are used, `PROG` and `_CLI_ZSH_AUTOCOMPLETE_HACK`. 
-Set `PROG` to the program name as before, set `_CLI_ZSH_AUTOCOMPLETE_HACK` to `1`, and 
-then `source path/to/autocomplete/zsh_autocomplete`. Adding the following lines to your ZSH 
-configuration file (usually `.zshrc`) will allow the auto-completion to persist across new shells:
+file included in this repo. One environment variable is used, `PROG`.  Set 
+`PROG` to the program name as before, and then `source path/to/autocomplete/zsh_autocomplete`. 
+Adding the following lines to your ZSH configuration file (usually `.zshrc`) 
+will allow the auto-completion to persist across new shells:
 
 ```
 PROG=<myprogram>
-_CLI_ZSH_AUTOCOMPLETE_HACK=1
 source  path/to/autocomplete/zsh_autocomplete
 ```
 #### ZSH default auto-complete example

help.go

@@ -102,7 +102,7 @@ func printCommandSuggestions(commands []*Command, writer io.Writer) {
 		if command.Hidden {
 			continue
 		}
-		if os.Getenv("_CLI_ZSH_AUTOCOMPLETE_HACK") == "1" {
+		if strings.Contains(os.Getenv("SHELL"), "zsh") {
 			for _, name := range command.Names() {
 				_, _ = fmt.Fprintf(writer, "%s:%s\n", name, command.Usage)
 			}