align columns

Implement http://rosettacode.org/wiki/Align_columns
Although in this specific program it doesn't change anything, I think it would be better to have `text: &str` instead of `text: &String`. If you do this, then you should also change the call site with `align_columns(text.as_slice())` instead of `align_columns(&text)`.

The reason taking a `&str` is better is that it makes your `align_columns` function more general. A `&str` is a slice, which is basically a pointer to an area of memory that can live anywhere. A &String is a pointer to an area of memory that lives specifically on the heap. Allocation on the heap is (relatively) expensive, so you want to avoid your caller having to allocate a String in order to call your function if they can.