Add bash/zsh shell completion

vintner completion bash|zsh prints a completion script meant to be
sourced (source <(vintner completion bash)); completes subcommands
(including short aliases), download's flags, and directory arguments
for install/env --bin. Mentioned in the top-level usage text and
documented in the README.

The Nivora package doesn't auto-install these system-wide yet - it'd
need Stapler's install-completion helper, whose calling convention
isn't documented anywhere in this repo or Nivora's other packages, so
guessing at it risked a broken package build for a nice-to-have.
source <(vintner completion bash) works today regardless of install
method (Nivora, prebuilt binary, or from source).

cmd/vintner/main.go

@@ -47,6 +47,8 @@ func runCLI(args []string) int {
 		return runInstall(args[1:])
 	case "env", "e":
 		return runEnv(args[1:])
+	case "completion":
+		return runCompletion(args[1:])
 	case "version", "v", "--version":
 		fmt.Println("vintner " + version)
 		return 0