Add a `:subject_interpolations` option to `ActionMailer::Base#mail`

It allows to have a properly scoped default I18n subject, but with interpolations, which
will be populated from the `:subject_interpolations` hash, e.g.:

    # config/locales/en.yml
    en:
      user_mailer:
        welcome:
          subject: 'Hello, %{username}'

    # app/mailers/user_mailer.rb
    class UserMailer < ActionMailer::Base
      def welcome(user)
        mail(subject_interpolations: {username: user.name})
      end
    end

I was also thinking of not introducing a new option, but use the `:subject` option, but detect if it's a Hash, and if so, pass it as interpolations to the default i18n subject.

As usual, open for feedback.