Add auto-generate dates for cloned graphics with slug without a date

[brmayes]

If you run the fab clone_graphic:SLUG,NEW-SLUG command with a NEW-SLUG without a date, it doesn't validate and return the NEW-SLUG with that day's date. Since we've implemented this validation in the fab create_graphic command and fab clone_graphic:SLUG command, it's worth keeping it consistent and adding the validation in this case as well.

[ghing]

This looks like a one-line fix, but testing whether this fix works or not is somewhat "expensive". At the very least, it will result in a directory that you need to cleanup in your local version of the graphics repo and a Google Spreadsheet that needs to be deleted.

This seems like a good opportunity to add some tests to this project and to learn about unit testing and mocking.

We should use pytest to write a test for the failing behavior for clone_graphic being sure to mock all the external APIs so we don't actually create files on the filesystem or Google Spreadsheets.

Next steps:

• Check out and step through the getting started section of the pytest docs
• Skim this article to get a better understanding of unit testing concepts. The examples use a different testing framework than the one we'll use, but it's still helpful.
• Read this article on using Python's mock package to avoid system calls. We'll be using mock to avoid actually calling things that alter the system or communicate with remote APIs.