Enable cropping via dynamic size array

In WP when you call `wp_get_attachment_image_src()` for example you pass in an array for the size parameter rather than a size name, in a Tachyon context this works great. There is however a limitation in WP that you cannot specify if the image should be cropped to the dimensions or not because all it does under the hood is find the nearest actual size or set width & height attributes on an image tag.

In Tachyon we can allow for overloading this size array with a 3rd item for cropping eg:

```php
$src = wp_get_attachment_image_src( $id, [ 300, 200, true ] );
```

This won't break any core code as it just references `$size[0]` and `$size[1]` directly but allows us to access `$size[2]` and choose whether to apply `resize` or `fit` in the tachyon URL.