Add .editorconfig to help enforce tabs for .js, spaces for .json

[fmoo]

See http://editorconfig.org/#overview for details, but tl;dr is this enables configured editors to insert tabs instead of spaces for .js and spaces instead of tabs for .json, to be consistent with current conventions, without requiring developers to override their global settings or enabling insecure local overrides.

[stefangimmillaro]

great addition yes