Added a check for the query URI parameter

With this addition, creating a simple HTML search form is possible.

```html
<form action="{{ base_url_relative }}/search" method="get">
    <input type="text" name="query" />
    <button>Search</button>
 </form>
```

Personally I think a 'query' URI parameter should take precedence over a path segment, but I'm not deep enough into Grav to make that call. So my preference would look like:

```php
$uri->query('query') ?: $uri->param('query');
```

As long as the user isn't doing something like `/search/query:ipsum?query=lorem` everything should be fine.