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

flag.go

@@ -21,9 +21,9 @@ var (
 	commaWhitespace = regexp.MustCompile("[, ]+.*")
 )
 
-// BashCompletionFlag enables bash-completion for all commands and subcommands
-var BashCompletionFlag = &BoolFlag{
-	Name:   "generate-bash-completion",
+// GenerateCompletionFlag enables completion for all commands and subcommands
+var GenerateCompletionFlag = &BoolFlag{
+	Name:   "generate-completion",
 	Hidden: true,
 }