Add _mailer suffix to generated file names.

It is somewhat confusing that [controllers](https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/controller/controller_generator.rb#L10) and [jobs](https://github.com/rails/rails/blob/master/activejob/lib/rails/generators/job/job_generator.rb#L19) follow this convention, but [mailers](https://github.com/rails/rails/blob/master/actionmailer/lib/rails/generators/mailer/mailer_generator.rb#L10) don't.