Add progress formatter

This is proposal to add progress formatter as a builtin formatter.

When I run RuboCop for projects which have few or no offences (like this project), I feel as if RuboCop is frozen because default formatter outputs almost nothing. So I've imeplemented progress formatter like RSpec's one.

It prints a mark for each file:
* No offence file: `.`
* Any offence file: highest severity code (like `C`, `W`)

There's an issue about this. `Eval` cop registers severity [`:security`](https://github.com/bbatsov/rubocop/blob/3bd91cffe40689e02e5814c2f4aedc40f362c35e/lib/rubocop/cop/eval.rb#L12), but it's not [predefined](https://github.com/bbatsov/rubocop/blob/3bd91cffe40689e02e5814c2f4aedc40f362c35e/lib/rubocop/cop/offence.rb#L8). Should we add `:security` to the predefined severities, or change `Eval`'s severity to `warning` or anything?