Allow arbitrary CLI arguments from rake task

[quiqueg]

The rake task should allow arbitrary CLI arguments. This feature is implemented for rubocop and scss-lint, and as far as I can tell it's necessary for specifying a formatter when running slim-lint via the rake task.

scss-lint implementation: sds/scss-lint@e2a96cb
rubocop implementation: https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/rake_task.rb#L57