Feature: Generators offer help/usage details

In order to reduce cost of learning about a new generator
As a generator user
I want help/usage details about generators

Scenario: List of visible generators for rubygems
  When I run local executable "rubigen rubygems" with arguments ""
  Then I should see "application_generator"
  Then I should see "component_generator"
  Then I should not see "migration" # from rails scope