Add "pending interactions" explanation to --progress formatter output.

[bethesque]

Need to subclass the RSpec Progress formatter and modify the output text to match lib/pact/provider/rspec/formatter_rspec_3.rb:71

Formatter is configured here:
lib/pact/provider/pact_spec_runner.rb:130

[bethesque]

1 interaction, 1 pending

Pending interactions: (Failures listed here are expected and do not affect your suite's status)

* A request for bar given bar exists

[tancnle]

@bethesque Happy to put up my hand on this.

[bethesque]

Ta! It's not as high a priority as v3 though. It's not a documented feature, and I only happened to realise it needed to be done because I had a left over example script from when I was seeing if the progress formatter option worked.