Ability to specify date format on input

Closes #16 
Could be something like:
```js
pad = (num, maxlen) => ((maxlen === 2 ? "0" : "00") + num).slice(-maxlen);
...
pad(5, 2);   // => 05
pad(50, 2); // => 50
pad(5, 3);  //  => 005
```