Make current_app customizable

Hi, thank you for providing such a nice gem :smile: 

I want to customize current_app name by ENV.

In my company, we have many apps on Heroku. They are running "production" environment but have different heroku app name (e.g. mycompany-myapp-servicename1 and mycompany-myapp-servicename2).

So I added a few lines. Please look a diff.

The order of getting current_app name is:

1. Try to get `ENV["MARCO_POLO_APP_NAME"]`. If it exists, use it.
3. If `ENV["MARCO_POLO_APP_NAME"]` is not defined, fallback to get current_app in normal way.

I've tested it on my personal Heroku app and it works well. I checked both `irb` and `pry`.

How do you think?