v0.15.0

• Add subcommand generate-manifest: write sorted name@version lines for all crates in a mirror (to a file or stdout)
• Add subcommand: cull : remove the crates listed in one or more manifests from a mirror, pruning now-empty directories,
  with --dry-run to preview
• Retry transient transport failures when downloading crates with backoff, and report non-200 responses as not found