Add formatter

[y-yu]

• qulacs has many C++ code which are NOT formatted like indentation or spacing....
  • Both tabs and spaces are used in the same file
• So I try to use C++ code formatter clang-format
• I add the clang-format setting file .clang-format
• Then I reformat using the command: git ls-files "*.cpp" | xargs clang-format -style=file -i
• I add check for violation of the format at .travis.yml
• Firstly I think it's enough to reformat indentation but its diff size is more than 10000..... 😇
  • It mixed some other reformatting(like spacing after commas)
  • You can see the differential ignored whitespace by https://github.com/qulacs/qulacs/pull/201/files?w=1
• And add .editorconfig to unite the coding rule

Discussion

• Would we need any formatter?
• Tab vs space?
  • Especially I don't care if either tabs or spaces but It could be a problem to include both in the same file or the same project
  • UPDATE: Some developers are using Visual Studio and it uses tabs for indentation. So I think it would be better to decide using tabs
    • As my research, Visual Studio uses spaces for indentation by default.... 🤔