Option to print status messages to stderr

[jeffparsons]

It would be nice for users to see an explanation of what's going on when Dotslash needs to download something.

Proposed design:

• Opt in per Dotslash file. (I.e. the author decides whether it's appropriate/safe for this binary.)
• Only ever write to stderr.
• Print only if it needs to download. And then print two messages:
  • One at the start. No details printed about where it's getting it from. No progress messages.
  • One when it's finished downloading and is about to exec the downloaded binary.

If the maintainers are open to this feature, I'd be happy to put up a PR implementing it.