Add .editorconfig file

[fktn-k]

This PR has added a .editorconfig file for the project so that file formats can be automatically decided/regulated according to the configurations in many popular text editors and IDEs.
If you're not familiar with EditorConfig, see the official site for detailed information.

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] The code coverage on your branch is 100%.
• [x] The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

coverage: 100.0%. remained the same when pulling 1d5bbe9d2f4e313b15e35c7c4eb0c28231337e0c on feature/add_editorconfig_file into fc9a99fbd632b1f3701d3083a1b0fd3fc855811d on develop.