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

this fixes #532
That would work.  I think it would be even better to make it a global config option for rspec-expectations; then we can have the `spec_helper` generated by `rspec --init` turn it on (so that new projects get it by default) with the plan to default it on in RSpec 4 and retire the option.  Thoughts?