Fix regression with semantic-logger, allow setting custom logger

[bkeepers]

This partially reverts the change in #489 to go back to using ActiveSupport::TaggedLogging instead of calling #tagged on an existing logger. However, now you can configure dotenv to use your own logger and it won't set up its own:

# config/application.rb

# Set a custom dotenv logger
Dotenv::Rails.logger = MyCustomLogger.new

module YourApp
  class Application < Rails::Application
    # …
  end
end

Fixes #493
cc @brosintoski @kriansa @alisaifee