File decodes based on extension, meaning it fails to load config if `.rc` files are used

[matronator]

I'm working on a CLI tool amock and I would love to be able to use config files with the rc postfix, like for instance .eslintrx or .zshrc or .npmrc, but when I try to load config from .amockrc it fails because it doesn't know it's a JSON file since it's missing the .json extension.

Would be nice to support this.