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

this fixes #532
I also had the concern that some 3rd party gems would have an issue if it was an all or nothing based on config value, but coming around to think about it that would be better, mostly because yes it would be annoying to have to include it any time, and especially 

> we should have included the chain clauses all along. It's a crappy user experience to force users to correct our mistake by manually calling a method on each matcher to make the matcher behave as it should have behaved all along.

It would be a nice experience for the user especially since they would
> get the benefit of this change without ever having to think about it.

Eventually these chained clauses should be expected to be included by default.  The configuration option could be clearly described in the documentation for creating custom matchers.

So based on the above we should:
* make a global config option for rspec-expectations, called *include-chained-matchers*
* have the spec_helper generated by rspec --init turn it on (so that new projects get it by default) 
* for RSpec 4, deprecate (or remove?) the option

Thoughts?  Does this sound good?