Add `_.existy` to define objects existance.

`existy` checks if `arguments` contains `undefined` or `null` values, if so (or if no arguments are passed) it returns false, otherwise it returns true.

Example:
```javascript
if (_.existy(iNeedToChekThis, andThis, andThat)) {
  // do something with those
}
```

I'm using lodash a lot both server and frontend side, I've created a small library to have `existy` and `truthy` as a help in some repetitive checks but I believe they belong in here. Credits to Michael Fogus for those two functions.

Please let me know if something is wrong in what I did or if it could be written in a better way.
If this is ok I would be happy to also add `truthy` to lodash.

Thanks for this project.