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

this fixes #532
While your original change to the `an_error_with_attribute` custom matcher above prompted the creation of this config option (as it helped us realize that this would be a breaking change!), at this point I'd prefer us to leverage the new feature internally rather than keeping our `description` override above and calling the `reset` method here.  We can set the config option to `true` in `spec_helper.rb` so it's on for our specs.