Add cache of found view files in FileViewFinder

Consider this example:

```html
<div class="users">
@foreach ($users as $user)
    @include('user.item', compact('user'))
@endforeach
</div>
```

For each user `View::make` is called and each time `ViewFinder` finds the view. More users we have, more times we find view that has been found already.

`FileViewFinder` now caches already found view files to fix this performance issue.