Add feature generator and include in list of available generators

I used to generate integration/request specs all the time from the command line. However, with the changes to Capybara, the DSL is only available in feature specs. So I have switched to using feature specs. After submitting a PR on the generator documentation, I realized feature specs could not be generated like integration/request specs. I wrote a feature generator that can be invoked from the command line and want to contribute so anyone else can use the generator if they like.