Add support for optionals

[gbooker]

I had a case where I needed to conditionally include members of a struct in json serialization based on certain criteria. Rather than create structs for each possible combination I wanted to indicate which fields should be present in the resulting json. Using std::optional is the obvious choice to indicate as to whether the field should be present in the json representation. Non-optional fields work as before.

Usage:

  struct OptionalFields
  {
    std::optional<int> count;
    std::optional<string> str;

    JSON5_MEMBERS(count, str);
  };

  struct OptionalArray
  {
    std::vector<OptionalFields> fields;

    JSON5_MEMBERS(fields);
  };

  OptionalArray array
  {
    {
      {1, {}},
      {2, ""},
      {{}, ""},
      {{}, {}},
      {5, "Golden Rings"},
    },
  };

  string json = json5::to_string(array);

results in:

{
  fields: [
    {
      count: 1
    },
    {
      count: 2,
      str: ""
    },
    {
      str: ""
    },
    {},
    {
      count: 5,
      str: "Golden Rings"
    }
  ]
}