Add flag for JSON with unquoted keys

[ChimeraCoder]

JSON keys are supposed to be quoted, though these are commonly omitted in practice. For example, the foursquare API documentation includes sample responses with unquoted keys, even though the actual responses themselves are valid.

While it's good that gojson only accepts JSON that can be handled correctly by encoding/json, it may be convenient to add an optional flag for this one case.