Child terms seem to come through as parent/child

Might need to add something like below to [this block](https://github.com/alleyinteractive/searchpress/blob/master/lib/class-sp-wp-search.php#L175).

``` php
if ( ! empty( $args['terms']['category'] ) && strpos( $args['terms']['category'], '/' ) ) {
$terms = explode( '/', $args['terms']['category'] );
$args['terms']['category'] = end( $terms );
}
```