Write a program that greets the user by name, or by saying "Hello, World!" if no name is given.
Challenge sourced from [http://exercism.io]
The `Hello World!` program will greet me, the caller.
If I tell the program my name is Alice, it will greet me by saying "Hello, Alice!".
If I neglect to give it my name, it will greet me by saying "Hello, World!"