New port: devel/picojson

PicoJSON is a tiny JSON parser / serializer for C++ with the following
properties:

• Header-file only
• No external dependencies (only uses standard C++ libraries)
• STL-friendly (arrays are represented using std::vector, objects are std::map)
• Provides both pull interface and streaming (event-based) interface

WWW: https://github.com/kazuho/picojson

PR: 237262
Submitted by: James Wright <james.wright@digital-chaos.com>
Reviewed by: tcberner, tobik
Differential Revision: https://reviews.freebsd.org/D19899