Refactoring names from bash to shell

The refactoring is required since completion is also supported for zsh
shell.

Refactoring details:
- flag generate-bash-completion -> generate-completion
- var  EnableBashCompletion     -> EnableShellCompletion
- var  BashComplete             -> ShellComplete
- var  BashCompletionFlag       -> GenerateCompletionFlag
- type BashCompleteFunc         -> ShellCompleteFunc

app.go

@@ -29,16 +29,16 @@ type App struct {
 	Commands []*Command
 	// List of flags to parse
 	Flags []Flag
-	// Boolean to enable bash completion commands
-	EnableBashCompletion bool
+	// Boolean to enable shell completion commands
+	EnableShellCompletion bool
 	// Boolean to hide built-in help command
 	HideHelp bool
 	// Boolean to hide built-in version flag and the VERSION section of help
 	HideVersion bool
 	// Categories contains the categorized commands and is populated on app startup
 	Categories CommandCategories
-	// An action to execute when the bash-completion flag is set
-	BashComplete BashCompleteFunc
+	// An action to execute when the shell completion flag is set
+	ShellComplete ShellCompleteFunc
 	// An action to execute before any subcommands are run, but after the context is ready
 	// If a non-nil error is returned, no subcommands are run
 	Before BeforeFunc
@@ -103,8 +103,8 @@ func (a *App) Setup() {
 		a.Version = "0.0.0"
 	}
 
-	if a.BashComplete == nil {
-		a.BashComplete = DefaultAppComplete
+	if a.ShellComplete == nil {
+		a.ShellComplete = DefaultAppComplete
 	}
 
 	if a.Action == nil {
@@ -136,8 +136,8 @@ func (a *App) Setup() {
 		}
 	}
 
-	if a.EnableBashCompletion {
-		a.appendFlag(BashCompletionFlag)
+	if a.EnableShellCompletion {
+		a.appendFlag(GenerateCompletionFlag)
 		a.appendFlag(InitCompletionFlag)
 	}
 
@@ -177,9 +177,12 @@ func (a *App) Run(arguments []string) (err error) {
 		return nil
 	}
 
-	var done bool
-	if done, err = checkInitCompletion(context); done {
-		return nil
+	if done, cerr := checkInitCompletion(context); done {
+		if cerr != nil {
+			err = cerr
+		} else {
+			return nil
+		}
 	}
 
 	if err != nil {
@@ -266,8 +269,8 @@ func (a *App) RunAsSubcommand(ctx *Context) (err error) {
 	a.Commands = newCmds
 
 	// append flags
-	if a.EnableBashCompletion {
-		a.appendFlag(BashCompletionFlag)
+	if a.EnableShellCompletion {
+		a.appendFlag(GenerateCompletionFlag)
 	}
 
 	// parse flags