Add support for warnings during rendering.

[tkaemming]

If this is used with toronado, it'd be helpful to trap all log messages routed to the toronado.cssutils logger (or all loggers, possibly?) so that warnings logged during rendering (such as for invalid CSS rules) are less likely to be overlooked.

[tkaemming]

Ideally, there would also be a hook so that other validation checks could performed (e.g. warn if assets are included via non-data: URL.)