Add two more tests for caesar per the changes to the spec for 2018, namely

* if the user provides no command-line arguments, or two or more, it prints Usage: ./caesar key. 
* checks to make sure that each character of that command line argument is a decimal digit (i.e., 0, 1, 2, etc.) and, if any of them are not, terminates (with a return code of 1) after printing the message Usage: ./caesar key.