If we are expanding these matchers to work elsewhere I'd rather have all configuration taken care either in its own file or required by the user in the test helper.
 
For rspec we could require the user to simply add this configuration to their spec_helper. 
For minitest is there any better way to handle registering methods, so we could take them out of this file?
  Are the classic assertions still highly used of could we get by without supporting them?