Refactor css_select: shorter and more readable

According to the documentation, `css_select` can only accept either
one or two arguments.

In the first case, the argument is always a string.
For instance: `css_select("div")`

In the second case, the last argument is always a string.
For instance, `css_select(form, "input")`

Therefore, the code can be simplified to take care of these cases
only, maintaining the specs as they are.