Introduce helper.config to allow overriding configuration

This introduces helper.config which gives some default config methods (currently only #code_size_thresholds) that can be used in rules.

These configs can be overriden by consumers with the following syntax:

helper.config do |config|
  config.code_size_thresholds = { high: 42, medium: 12 }
end

This takes a very simple and naive approach at the moment.

Closes #6 (closed).