Add truncate & truncateFromCenter with test

This should be better right now :smile: 
This looks better! However, it's not exactly the behavior I would expect. I think that the most important feature of `truncate()` is that it always returns a string of the length you specify, even if that means ditching the ending. So "foo" at length 1 should return "f" and at length 3 should return "foo". Actually, the first three test cases of `truncateFromCenter()` all apply here.

Let's also line things up here so it's a bit easier to read. That means that all the expected values should be indented to line up together. Leave the comma attached to the expected method call.