Support replacer parameter in JSON5.stringify()

Hi,

I spotted this project on 24 Pull Requests, and it looked cool! I've had a go at resolving issue #38 (i.e. supporting 'replacer' params in JSON5.stringify() calls), since it was marked as pull-requests-welcome.

I've done my best to cover the test cases I could think of, but I'm willing to believe there's something I've missed, so please shout if so.

I've also branched of develop - I wasn't sure if that was correct or not. I can rebase my changes on master if that's preferable?

Thanks,
Rowan