Add generative binary, update README

After trying to explain the arguments that the `rspec` command needs in the
README, I realized that we could simply encapsulate them in a `generative`
command. This should allow someone to add `gem 'generative'` to a project, and
then simply run the `generative` command. RSpec will still need `generative`
required to avoid a `NoMethodError` for the `generative do/end` block.