option to throw on duplicate keys for .parse

[silverwind]

RFC7159 says:

The names within an object SHOULD be unique

Despite that, pretty much all implementations of JSON.parse handle them gracefully, which creates an issue when one tries to be compatibly with YAML where duplicate keys are generally a fatal error.

I think it'd be great if this parser had an option to be strict on duplicate keys.

[prantlf]

I opened PR #31 as a suggested fix for this issue.