Move the singleton methods in Config to new class ConfigLoader.

Conflicts:
	lib/rubocop/cli.rb
	spec/rubocop/cli_spec.rb
Refactor some unnecessarily long methods.
Ignore lines of inner classes/modules in ClassLength
Allow namespace class without documentation in Documentation cop
Merge pull request #547 from jonas054/config_loader

Extract ConfigLoader class
Merge pull request #550 from yujinakayama/handle-namespace-class

Handle namespace classes in ClassLength and Documentation