Added ugly ES5 equivalent of findIndex

[jurosh]

I was looking into finding index in array by value - and thinking if it's not possible to do functional way.
So I created this example and I think it's equivalent for ES6 findIndex feature, even though it's so ugly, but it will find the index.

[ 1, 3, 4, 2 ].map(function(x, index) { return x > 3 ? index : undefined; }).filter(function (x) { return x !== undefined; })[0];

-- So everyone can see how easily it's now done in ES6

[akhoury]

findIndex accepts a test function.

Also you don't need to iterate twice, see the polyfill for example.