Description:
    Generates a new mailer and its views. Passes the mailer name, either
    CamelCased or under_scored, and an optional list of emails as arguments.

    This generates a mailer class in app/mailers and invokes your template
    engine and test framework generators.

Examples:
    `bin/rails generate mailer sign_up`

    creates a sign up mailer class, views, and test:
        Mailer:     app/mailers/sign_up_mailer.rb
        Views:      app/views/sign_up_mailer/signup.text.erb [...]
        Test:       test/mailers/sign_up_mailer_test.rb

    `bin/rails generate mailer notifications sign_up forgot_password invoice`

    creates a notifications mailer with sign_up, forgot_password, and invoice actions.