Allow JSON comments

[luposlip]

Currently, parsing JSON with comments will fail (and it's illegal according to the JSON specification).

But that's nuts. We like comments in our code, and in our data as well.

@technomancy reminded me of this today, with his post on that exact issue:
https://douglascrockfordisnotyourdad.technomancy.us/

I'll find a way to make it work, eventually. Probably as a flag that has to be set, to allow comments (not to break existing use of this library).