Add .editorconfig and .clang-format files to help with consistent formatting by multiple contributors

[lewissbaker]

From comment in #45.

[lewissbaker]

Probably also want to run clang-format over the code to get a consistent baseline style once these have been added.

[lewissbaker]

Commit bdd7ccc88d8fce7c4543d3c153df430b6dc789a9 adds initial versions of .editorconfig and .clang-format

These should most-closely mirror current style. I'll apply clang-format to existing files after resolution for #45 has been merged to avoid unnecessary conflicts.