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.