Add mapping function as optional argument to sift

[aleclarson]

I like sift, but sometimes I want to filter based on some property or other value derived from each array item.

// Filter out objects with a falsy "foo" property.
sift(arrayOfObjects, obj => obj.foo)

If we do this, #36 will make sift even better.

// Filter out objects with a falsy "foo" property.
sift(arrayOfObjects, 'foo')