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

this fixes #532
`include_chain_clauses_in_custom_matcher_descriptions`, sounds better.

As far as the configuration of this value, the natural inclination would be for it to be in RSpec::Expectations::Configuration since its value is consumed in this library.

But if rspec-core is generating spec_helper, and setting a value for this, shouldn't the config for this be locate din the Core configuration, even though it's not consumed in that library?  That feels a bit weird, right?