Optionally enable relaxed JSON parsing in json_parser

[prat0088]

I'm using json_parser and we have many, many files that have trailing commas. This breaks the current implementation because the official JSON spec does not prohibit trailing commas. Rather than write a custom parser or impose a change on our developers, it would be nice to have an option to enable relaxed mode.

[iafan]

I decided not to introduce any options, and just support relaxed parsing by default. This is backwards compatible.