Drop -s/--silent option

This is a proposal to drop `-s/--silent` option.

With this change, `progress`/`simple`/`clang` formatters always report summary and `emacs`/`files` formatters no longer report.

As I mentioned this [before](https://github.com/bbatsov/rubocop/issues/222#issuecomment-18716079), `-s/--silent` option was introduced just for `-e/--emacs` option by #21. However currently we have more formatters and I think the meaning of _summary_ is ambiguous.

I thought I should deprecate first before dropping, but I guess the release of 1.0 is soon and actually this change does not break almost external tools. Instead I made RuboCop ignore `-s/--silent` option not to raise error.