Add transparent comparator and perfect forwarding support to find() and count()

Already covered by AppVeyor

Instead implement @gregmarr's PR comments that perfect forwarding should be used. Also cleaned up cpp language standard detection.

Hopefully, #764 fixed this.

Simplified istream handing #367

Copied from solution to https://github.com/nlohmann/json/issues/464

This avoids unnecessary string copies on often used
find().

Error : 'identifier "size_t" is undefined' in linux

Fix Visual Studio 2017 warnings

Fix warning C4706 on Visual Studio 2017 - fixes #784

Set GENERATE_TAGFILE in Doxyfile

Lots of tests have this warning.

Also moved out of for loop, doesn't need to be done every loop.

This causes test-udt to crash due to bad iterator

Allows documentation to be linked from other projects to https://nlohmann.github.io/json using Doxygen with the tag:
TAGFILES = $(SOME_PATH)/nlohmann_json.tag=https://nlohmann.github.io/json

o For some unknown reason, the complexity of the benchmark platform
  prevented some C++ compilers from generating optimal code, properly
  reflective of the real performance in actual deployment.
o Added the json_benchmarks_simple target, which performs the same
  suite of tests as json_benchmarks.
o Simplified the benchmark platform, and emit an "Average" TPS
  (Transactions Per Second) value reflective of aggregate parse/output
  performance.