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

this fixes #532
I think most users will configure this via the rspec-core config API:

``` ruby
RSpec.configure do |config|
  config.expect_with :rspec do |c|
    c.include_chain_clauses_in_custom_matcher_descriptions = true
  end
end
```

...and I'd prefer that was used here as well.