[c++] coding convention

http://llvm.org/docs/CodingStandards.html

https://github.com/bloomberg/bde/wiki/physical-code-organization

https://webkit.org/code-style-guidelines

https://github.com/facebook/hhvm/blob/master/hphp/doc/coding-conventions.md

https://google.github.io/styleguide/cppguide.html