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

this fixes #532
I don't think "expectation" is the right term here.  (In this library, we consider an expectation to be a whole `expect(...).to matcher` expression).  Also, `RSpec::Matchers.configuration` isn't the normal API we recommend using for setting config (see [our docs](http://rubydoc.info/github/rspec/rspec-expectations/RSpec/Expectations/Configuration)) -- it's really just there because rspec-core needs it for it to work with `expect_with(:rspec) { }`.  I think I'd rephrase this as something like:

``` ruby
# If the method is called, and the `include_chain_clauses_in_custom_matcher_descriptions`
# config option has been enabled, then the default `description` and `failure_message`
# will include a corresponding English phrase based on the method name and arguments.
```