Import/Export compressed JSON files #1556

I've found in the doc it's possible with this library dealing with binary JSON formats (BSON, CBOR, etc.). Anyway looking around the internet one of the most efficient ways of storing JSON files, in terms of space consumption, seems to be using standard JSON format and compress the file in a .json.gz or .json.bz2 file (as shown here and here). I understand this can be done manually with few commands and it could require to add an undesirable dependency of your json.hpp on an external compression library. On the other hand I think it would be a useful nice-to-have doing it with a single line as it is very common using compressed formats for JSON files especially in network data transmission.