Adding a little helper to check if an element/selector exists

Every time I start a project and after about 3 times writing:

```js
if ( $(".unicorns").length > 0 ) {
    $(".rainbows").show();
}
```
I get mightly irked and end up writing a `$.fn.exists()` function so... never again!!!
If it is too simple for the library I understand; but I felt a contribution itch and needed to scratch it. :smile: