When adding this library as embedded library with private
"target link", e.g. only used inside private source files, the
library does not need to be installed when the main project gets
installed.

This adds an additional option `JSON_Install` similar to the
test-build control switch in order to skip installing headers and
CMake config files if requested.

Avoids using
```cmake
add_subdirectory(path/to/nlohmann_json EXCLUDE_FROM_ALL)
```

which has further side-effects:
https://cmake.org/cmake/help/v3.0/command/add_subdirectory.html

Properly convert constants to CharType

Add macro to define enum/JSON mapping

Add BSON support

- added fuzz testers
- added some reference files
- made an exception text more clear

Merge branch 'feature/bson' of https://github.com/julian-becker/json into julian-becker-feature/bson

Allow to set error handler for decoding errors

Thanks @abolz!