Add support for "secret" command namespace

[kkirsche]

Fix #141

Adds support for secret namespace:

 ~/git/go/src/github.com/twpayne/chezmoi >> enhancement/secret_toplevel $ go build -o cmoi
 ~/git/go/src/github.com/twpayne/chezmoi >> enhancement/secret_toplevel $ ./cmoi secretInteract with a secret manager

Usage:
  chezmoi secret [command]

Available Commands:
  pass        Execute the pass CLI
  vault       Execute the Hashicorp Vault CLI

Flags:
  -h, --help   help for secret

Global Flags:
  -c, --config string        config file (default "/Users/kkirsche/.config/chezmoi/chezmoi.yaml")
  -D, --destination string   destination directory (default "/Users/kkirsche")
  -n, --dry-run              dry run
  -S, --source string        source directory (default "/Users/kkirsche/.local/share/chezmoi")
  -u, --umask int            umask (default 022)
  -v, --verbose              verbose

Use "chezmoi secret [command] --help" for more information about a command.
 ~/git/go/src/github.com/twpayne/chezmoi >> enhancement/secret_toplevel $ ./cmoi secret vault -h
Execute the Hashicorp Vault CLI

Usage:
  chezmoi secret vault [flags]

Flags:
  -h, --help   help for vault

Global Flags:
  -c, --config string        config file (default "/Users/kkirsche/.config/chezmoi/chezmoi.yaml")
  -D, --destination string   destination directory (default "/Users/kkirsche")
  -n, --dry-run              dry run
  -S, --source string        source directory (default "/Users/kkirsche/.local/share/chezmoi")
  -u, --umask int            umask (default 022)
  -v, --verbose              verbose