ARDUINOJSON_ENABLE_STD_STRING

Version 6 is currently in beta, so the API ~~may~~will change in the next revisions.

Unless you need the new features of version 6 (e.g. MessagePack), please use version 5.
Version 6 should stay in beta stage until 2019.

Enables support of the type std::string in the library.

Default is 0 of ARDUINO is defined, 1 otherwise.