Is your feature request related to a problem? Please describe.
I believe there are several issues with the current standard of the config file being .all-contributorsrc in the root of the repo:

1. Lack of extension makes the format ambiguous to some editors
2. Config files in the root clutter things up and make the repo less readable, in my opinion

Describe the solution you'd like
I would propose supporting the following configs, with the CLI using the first one detected:

• .all-contributors.json
  • This could potentially become the new standard recommended in the docs, also?
• .config/all-contributors.json
• .all-contributorsrc

Describe alternatives you've considered
Question not applicable.

Additional context
Adding support for .config directory is inspired by https://github.com/pi0/config-dir, a growing movement to try and organize config files in a set directory rather than the repo's root.

Additionally, I would be happy to contribute a PR for this is there is interest from the maintainers.

Thanks!