Added support for center alignment

I'm not familiar enough with the multi-line and truncating options, so I haven't tested those.

If this works well, it should close https://github.com/timoxley/columnify/issues/4
this:
```javascript
Array.apply(null, {length: Math.ceil(len/str.length) + 1}).join(str).slice(0, len)
```
would be more correct and efficient, but since str is usually of length 1 or very very short, it is probably better to accept the minimal performance penalty for clearer code.