Move most of spec runner's state into Spec::CLI

[HertzDevil]

This PR combines most of Spec's global state into a new Spec::CLI class, with the end goal of making the spec runner itself more reusable and testable. The new singleton state is Spec.cli; eventually, only Spec's public class methods should delegate to this singleton, and methods within the runner shouldn't refer to it. The remaining class variables are:

• Spec::RootContext.@@instance
• Spec::RootContext.@@current_context
• Spec::RootContext.@@spec_nesting
• Spec::Example.@@example_counter
• Spec.@@lines_cache

Class methods of Spec that should have been nodoc are marked as deprecated.

[Fryguy]

Love this! I've had this on my TODO for so long but haven't had time. Thanks for digging into it!