Added configuration file for EditorConfig.

This is a universal configuration file for common editor settings such as
indentation sizes. Since we're using 2 spaces for C/C++ this should keep things
consistent (given users have an EditorConfig plugin installed). For example, in
my own case Vim keeps defaulting to 4 spaces which is rather annoying.

See http://editorconfig.org/ for more information about EditorConfig itself.