Adds comet_ml to the list of auto-experiment loggers
This PR does three things:

* abstracts the auto-experiment-loggers' is_available() functions into an "integrations.py" file
* adds comet_ml to the list of auto-loggers available
* updates and reorganizes the docs slightly 
@LysandreJik Will do! When I run `make style` it makes corrections to many other files, and also (for example) changes the import sort order on code that I didn't touch in the code that I am editing. Am I doing something wrong? Or should I commit those changes (in the files I am editing)?<|||||>Sounds weird, maybe you're missing some packages in your environment. Let me know when you're finished and I'll push the styling on your branch.<|||||>@sgugger Should have all of the review comments addressed. Thanks to everyone!<|||||>Great, thanks for iterating @dsblank!<|||||>You're welcome, and thanks for all of the work on this project! Looking forward to productive ML!