New interface to specify multiple formatters

This is the implementation of my proposal for a new multiple formatter interface:

```ruby
SimpleCov.formatters = [
  SimpleCov::Formatter::HTMLFormatter,
  SimpleCov::Formatter::CSVFormatter,
]
```

This is the current interface:

```ruby
SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
  SimpleCov::Formatter::HTMLFormatter,
  SimpleCov::Formatter::CSVFormatter,
]
```

I think my proposal is better because, conventionally, the `[]` method is used to fetch a value, not to set a value.

This proposal would not require breaking the current interface `SimpleCov.formatter` because I’ve proposed adding a new method `SimpleCov.formatters` (with an “s”).

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.