Add documentation for integration with CMake and hunter

https://github.com/nlohmann/json/pull/663#issuecomment-318931884

REFACTOR: rewrite CMakeLists.txt for better inlcude and reuse

Support moving from rvalues in std::initializer_list

- replaced list of pairs by flat list in next_byte_in_range
- implemented early exit in case of parse errors
- reused memory for object keys
- direct calls to embedded objects/arrays for insertions

The rewrite uses more cmake build-in automatisms and build-in generates
variables to allow better generic reuse.
* cmake  files are installed to
``` <install_prefix>/lib/cmake/nlohmann_json/ ``` for best support on
most systems
* include path is set to ``` include ```  for usage as ``` #include
<nlohmann/json.hpp> ```

enable_testing only if the JSON_BuildTests is ON

You can now pass a boolean "allow_exceptions" to the parse functions. If it is false, no exceptions are thrown in case of a parse error. Instead, parsing is stopped at the first error and a JSON value of type "discarded" (check with is_discarded()) is returned.

On MSVC compiler, temporaries that are constructed during a
list initialization, are sometimes destroyed even before calling
the initializing constructor, instead of at the end of the
containing full-expression. This is clearly non-conforming to
[class.temporary].
As the impact of this bug is silently producing incorrect
JSON values, move eagerly from rvalues to be safe.

See https://stackoverflow.com/questions/24586411

C++ overload resolution/list initialization rules are hard.

- You can now pass a reference to a vector to the to_cbor and to_msgpack functions. The output will be written (appended) to the vector. #476

- You can now pass an output stream with uint8_t character type to the to_cbor and to_msgpack functions. #477

- You can now read from uint8_t */size in the to_cbor and to_msgpack functions. An input adapter will be created from this pair, so you need to use braces. #478

std::initializer_list does not own the temporaries created in
its initialization. Therefore, storing it in an independent
stack variable is unsafe.

This commit works around an issue in std::initializer_list design.
By using a detail::json_ref proxy with a mutable value inside,
rvalue-ness of an input to list initializer is remembered and
used later to move from the proxy instead of copying.

We now rely on implicit conversions to an input_adapter object in the parse/accept functions.

Travis found an error with Clang 3.8's sanitizers, see https://travis-ci.org/nlohmann/json/jobs/256366699. Unfortunately, I cannot reproduce this error with clang version 6.0.0 (trunk 308825) locally. However, this seems to be an issue, because so far, we did not reset a value after moving from it.

Internally, the parser now writes its result into a JSON value provided as a reference. To be usable, the public interfaces need to be extended.

- Replaced codepoint calculation by bit operations.
- Fixed several clang-tidy warnings.

A complete rewrite of the string escape function. It now provides codepoint-to-\uxxxx escaping. Invalid UTF-8 byte sequences are not escaped, but copied as-is. I haven’t spent much time optimizing the code - but the library now agrees with Python on every single Unicode character’s escaping (see file test/data/json_nlohmann_tests/all_unicode_ascii.json).

Other minor changes: replaced "size_t" by "std::size_t"

Rename BuildTests to JSON_BuildTests

added Contributor Covenant Code of Conduct