Update multiple formatter interface

I left the decision to deprecate `SimpleCov::Formatter::MultiFormatter.[]` up to the maintainers. IMHO, a version of `simplecov` should be released with a much simpler interface. For example, I find it a bit strange that `SimpleCov.formatter` is both a setter and a getter, when there is also an explicit `SimpleCov.formatter=` attribute writer. I’m not sure why the setter needs to be spelled two different ways.