Add max() and min()

Add function to get max() and min() values of a collection

```
<TKey> T max(Selector<T, TKey> selector, Comparator<TKey> comparator);
<TKey extends Comparable<TKey> T max(Selector<T, TKey> selector);

<TKey> T min(Selector<T, TKey> selector, Comparator<TKey> comparator);
<TKey extends Comparable<TKey> T min(Selector<T, TKey> selector);
```
