add chained expectation descriptions to the default descriptions of custom matchers with fluent interfaces

this fixes #532
@myronmarston Cannot figure out how to cleanly set config this way, then during cleanup revert it back to its default .  I can't seem to find a spec example where configuration is modified as in the [example you sent](https://github.com/oveddan/rspec-expectations/blob/320063ba256150da799b9019a22c7bf2a0ce9503/spec/rspec/expectations/configuration_spec.rb#L6) and then the behavior of code outside of Configuration is affected.  Can you point me to one?