Future - Change default behavior of merge

[bebraw]

To quote @sapegin, a better merge default might be to "not add duplicates but also do not remove existing items."

Something along:

const merge = mergeBase({
    customizeArray: (a, b) => a.concat(differenceWith(b, a, isEqual)),
});

[bebraw]

It likely doesn't make sense to change this as the basic primitives in #135 feel strong enough. In particular unique solves the problem of duplicates.