title: How to pass flag values to subcommands in golang urfave cli
body: <p>I am using urfave at <a href="https://github.com/urfave/cli" rel="nofollow noreferrer">https://github.com/urfave/cli</a>
  to create a CLI with two subcommands.
  I am able to create a CLI with a subcommand,
  but I really have no idea how to define the flags.
  What's the difference between the global flag and local flag?</p>