Deprecate --format disabled and add cop DisableCopComment

The `--format disabled` option was implemented in #900. We (@fshowalter, @bbatsov, @yujinakayama and myself) had some discussion about it then, but I finally gave in and said that it was okay. I now think that I was wrong to do so, and that my initial comment about implementing the check as a cop was one that I should have fought harder for. Let me show you why.

### Problems with the current implementation

The `disabled` formatter doesn't behave like a formatter. Instead of only controlling how the result is reported, it reports something different than the other formatters. If there are offenses, they are reported by all the other formatters, but not by `-f d`. The cops are still run, even if `disabled` is the only formatter given, but there's no reporting of their findings.

So the list of `rubocop:disabled` comments can only be reported in one format. We talked in #900 about the possible need to get the output in JSON format. I think I forgot to follow up on that concern in our discussion then, so that's not possible now.

There's something strange in the current reporting and its use of `Infinity`. If the file is not infinitely large, I think it's untrue to say that the range doesn't end. :smile: No-one has complained about this. Is that because the formatter isn't used much?
```
$ ./bin/rubocop -f d lib/rubocop/formatter/

Cops disabled line ranges:

lib/rubocop/formatter/base_formatter.rb:3..44: Metrics/LineLength
lib/rubocop/formatter/base_formatter.rb:3..Infinity: Lint/UnusedMethodArgument
```

### Why a cop is better
Aside from avoiding the problems listed above, a cop solution where the cop is *enabled* by default also has the benefit of putting the offense count in `.rubocop_todo.yml` when `--auto-gen-config` is run.

The new code is smaller than the old. That's nice in itself, but I also think that it's cleaner to not send the disabled line ranges to all the fomatters in [`runner.rb`](https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/runner.rb#L158). Not a big deal, but it caused a problem for me in another PR I'm working on.

### What's missing
We don't get the reporting of line ranges with the cop. Admittedly that's a deterioration compared to the current functionality. [**Edit:** This is no longer the case. See further down.]