Fixes #2315

This is attempt to fix #2315 by supporting passing any operator as the first argument:

```php
$query = (new Query())->where(['<=', 'a', 'b']);
// results in WHERE "a" <= "b"
```

The pro of this solution is that any operator is supported. The con is that operator isn't whitelisted anymore and since it's not escaped, it may result in SQL injection of taken from user input.

**TODO**

- [x] Decide if it should support any operator or there should be a whitelist.
- [x] Guide.
- [x] Changelog.