Update to new JSON library

[finger563]

much better use in code, see here

[AWatk]

https://github.com/nlohmann/json#tofrom-strings

allows the same user configuration setup to work with the new library

[AWatk]

Basic usage section explains the importance of using the at() function instead of the [] operator for value retrieval. at() will throw an exception instead of giving undefined behavior in the case that a key does not exist.

https://github.com/nlohmann/json#basic-usage

[p-ranav]

I'm glad y'all are moving to this.

[finger563]

*we're - FTFY

[AWatk]