Allow IndentationWidth cop to ignore certain lines from indentation

See: bbatsov/rubocop#1031
I think you should add an empty array instead of having an optional parameter. Then it will be consistent with:
```yaml
Style/GlobalVars:
  AllowedVariables: []
```
And if you do that, you won't have to check if it's `nil` in `IndentationWidth#configured_excluded_keywords`.