Add --init-completion flag to print completion code

This flag takes as input 'bash' or 'zsh' and generates the completion
code for the specified shell.

app.go

@@ -138,6 +138,7 @@ func (a *App) Setup() {
 
 	if a.EnableBashCompletion {
 		a.appendFlag(BashCompletionFlag)
+		a.appendFlag(InitCompletionFlag)
 	}
 
 	if !a.HideVersion {
@@ -176,6 +177,11 @@ func (a *App) Run(arguments []string) (err error) {
 		return nil
 	}
 
+	var done bool
+	if done, err = checkInitCompletion(context); done {
+		return nil
+	}
+
 	if err != nil {
 		if a.OnUsageError != nil {
 			err := a.OnUsageError(context, err, false)

flag.go

@@ -27,6 +27,12 @@ var BashCompletionFlag = &BoolFlag{
 	Hidden: true,
 }
 
+// InitCompletionFlag generates completion code
+var InitCompletionFlag = &StringFlag{
+	Name:  "init-completion",
+	Usage: "generate completion code. Value must be 'bash' or 'zsh'",
+}
+
 // VersionFlag prints the version for the application
 var VersionFlag = &BoolFlag{
 	Name:    "version",

help.go

@@ -286,3 +286,41 @@ func checkCommandCompletions(c *Context, name string) bool {
 
 	return false
 }
+
+func checkInitCompletion(c *Context) (bool, error) {
+	if c.IsSet(InitCompletionFlag.Name) {
+		shell := c.String(InitCompletionFlag.Name)
+		progName := os.Args[0]
+		switch shell {
+		case "bash":
+			fmt.Print(bashCompletionCode(progName))
+			return true, nil
+		case "zsh":
+			fmt.Print(zshCompletionCode(progName))
+			return true, nil
+		default:
+			return false, fmt.Errorf("--init-completion value cannot be '%s'", shell)
+		}
+	}
+	return false, nil
+}
+
+func bashCompletionCode(progName string) string {
+	var template = `_cli_bash_autocomplete() {
+     local cur opts base;
+     COMPREPLY=();
+     cur="${COMP_WORDS[COMP_CWORD]}";
+     opts=$( ${COMP_WORDS[@]:0:$COMP_CWORD} --generate-bash-completion );
+     COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) );
+     return 0;
+};
+complete -F _cli_bash_autocomplete %s`
+	return fmt.Sprintf(template, progName)
+}
+
+func zshCompletionCode(progName string) string {
+	var template = `autoload -U compinit && compinit;
+autoload -U bashcompinit && bashcompinit;`
+
+	return template + "\n" + bashCompletionCode(progName)
+}