Additional Config File Options

**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!