feat: allow extra hooks in local config

[fabn]

Without this patch is not possible to define extra hooks in local config, e.g.

# lefthook.yml
tests:
  commands:
    tests:
      run: go test ./...

and

# lefthook-local.yml
lints:
  scripts:
    "linter.sh":
      runner: bash

Won't work, it wont be possible to run lefthook run lints

⚡ Summary

Iterate against both global and local top level keys when adding custom hooks.

☑️ Checklist

• Check locally
• Add tests

[mrexox]

Thank you for bringing this feature! I will release a new version this week.