Rework internal flatten function

http://jsperf.com/new-flatten

For lodash speeds we'll need to change `push.apply` to something similar to [this clever logic](https://github.com/lodash/lodash/blob/master/lodash.js/#L1616-1619). Let me know if that's more desirable, I have it [already prepared](https://github.com/megawac/underscore/blob/flatten2/underscore.js#L465-483) and its in the jsperf
if `value.length` is `0` then it will go to the `} else if (!strict) {` which means it could end up pushing an empty array to `output`.