Simplified handling of enabled flag

Handling of enabledness has been moved out of the `Cop` class and is now done only in `cli.rb`. Based on @yaauie's [comment](https://github.com/bbatsov/rubocop/pull/10#issuecomment-12334060) in discussion about `configurable` branch.

Cops are instantiated only if they're enabled. This doesn't produce any noticable speed increase, but is a simplification IMHO.