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

this fixes #532
> Except these things are usually controlled by the team and not an individual and have to be consistent across projects that may have differing RSpec versions.

Do you think this is a likely case?  If so, and the value is configured, the libraries with the out of date rspec versions would raise an error since they would not be aware of that configuration method.