Allow .json file extension

[spazzylemons]

In certain circumstances (for me, that would be using Neocities Free which has a file extension whitelist), one may want or be required to have a file extension for files on their domain. I suggest that .well-known/fursona.json be allowed as a fallback in case .well-known/fursona does not exist.

[pyrox0]

That's a good idea, I'll add that to the readme!