Add `envchain --list [namespace]`

- Without namespace, envchain shows list of namespaces
  * it searches keychain items where item description is 'envchain'.
  * Before d7e573145ad4ea8ca71312397b0a06b96575833c, doesn't set
    item description. So we have to set description manually for older
    items.
- With namespace, envchain shows lists of environment variable names in
  the specified namespace. You can use `--show-values` (`-v`) option to expose
  values too.