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

this fixes #532
> make a global config option for rspec-expectations, called include-chained-matchers

Yep.  I don't think `include_chained_matchers` is a clear name in the context of the config object, though.  How about `include_chain_clauses_in_custom_matcher_descriptions`?

> have the spec_helper generated by rspec --init turn it on (so that new projects get it by default)

Yep.

> for RSpec 4, deprecate (or remove?) the option

We'll probably do what we did for rspec-core's `treat_symbols_as_metadata_keys_with_true_values` option: make it a no-op in RSpec 4 (and have it print a deprecation warning) and remove it in RSpec 5.