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

this fixes #532
I was not sure of how to access the expectation configuration - thanks for showing me.  

My attempts at doing:

```ruby
RSpec.configure do |config|
  c.include_chain_clauses_in_custom_matcher_descriptions = true
end
```

were raising an error, but now I see the correct way of accessing the expectation configuration.