[Optional] Add current date/timestamp to the performance_report() context manager

Hello team.

I'm currently getting my feet wet with Dask and atm I was doing some **performance_report()** creation with the default settings. And I have found myself concatenating the current date to my `filename` variable from time module. It looks smth like this:

> time.strftime("%b_%d_%Y_%H_%M_%p", _**time.gmtime()**_)

Please consider adding the timestamp to the default filename. Maybe check if the filename='dask-report.html' and if positive use the snippet above.