Description:
    Generate a new campaign

Example:
    rails generate heya:campaign Onboarding first:0 second:2.days third

    This will create:
        app/campaigns/onboarding_campaign.rb
        app/views/heya/campaign_mailer/onboarding_campaign/first.text.erb
        app/views/heya/campaign_mailer/onboarding_campaign/first.html.erb
        app/views/heya/campaign_mailer/onboarding_campaign/second.text.erb
        app/views/heya/campaign_mailer/onboarding_campaign/second.html.erb
        app/views/heya/campaign_mailer/onboarding_campaign/third.text.erb
        app/views/heya/campaign_mailer/onboarding_campaign/third.html.erb