Refactoring Sandboxing to be in RSpec support

rspec-core changes for https://github.com/rspec/rspec-core/issues/1798

rspec-support changes in https://github.com/rspec/rspec-support/pull/137
While these config values are the ones we've wanted for rspec-core's specs, I don't think it makes sense to mutate the config object at all in the publicly exposed sandbox.  Maybe we can see if they aren't actually needed for rspec-core's specs?  Or, if they are, they can go in rspec-core's spec sandbox on top of the public sandbox you are providing here.