Configuration API and thread safety

Configuration options can now be made globally, e.g.

RegexpExamples::Config.max_repeater_variance = 5
RegexpExamples::Config.max_group_results = 10
RegexpExamples::Config.max_results_limit = 20000

Or within a block, e.g.

RegexpExamples::Config.with_configuration(max_repeater_variance: 5) do
  # ...
end

All forms of configuration are now thread safe.