Add string formatting support to log

I see from #33 that this logger will be deprecated soon, but even so it was pretty trivial to add string formatting support since node’s `console.log` already pipes its input through `util.format`.

Ex:
```javascript
log('%s + %s + %s =', 1, 2, 3, 'five');
// => [02:32:01] 1 + 2 + 3 = five
```