Lost precision in double values

[guillaumechereau]

I would like to use json-builder for a project that sometime has to export time in julian date (as a number of day + fraction of day). The values need to keep a high precision (at least 6 digits after the coma), but currently json-builder doesn't handle this case very well, because the '%g' format used in the code will for example convert "55080.708333" to "55080.7"

I wish there was an option to specify the format used. Maybe a string in json_serialize_opts to override the "%g" ?

[LB--]

There was some discussion about this in #6, I agree it would be a good option to have. Being able to perfectly round-trip the data is important and should be achievable.