Moving argument handling from if statements to functions

If you take a look at my not-yet-PR'd branch [handlerRefactor](https://github.com/Steampunkery/py-todo/tree/handlerRefactor), you'll see the general idea for what I'm thinking. The long if statement we have now is not very pretty or easy to read. I think it would be a good idea to move all that functionality from the if statements into functions. As you can see from my branch, each function is then registered into a dictionary. They key in the dict is a tuple of command line arguments that trigger the given function, which is stored as the value in the dict.

It might even be a good idea to put the handler functions in another file in the future. I did some research and it's possible to bundle multiple python files into a single executable zip file, so you won't need to install multiple files, just zip them all together.