Add .editorconfig for the basic code style consistency

[halfzebra]

@tcoopman it's totally up to you whether you want it in the project or not, but the code style deserves a little love 🙂

I didn't fix the existing code style inconsistency with mixed tabs/spaces to not screw the Git history for no reason.

[tcoopman]

I've never used editorconfig and it doesn't look that useful for this kind of project. I would rather add a prettier task in package.json if we'd like to enforce the style. I changed the double quote manually.

Thanks for the improvements.