Add option to the Kraken CLI to show Python warnings

[NiklasRosenstein]

Having an option to enable warnings (see snippet below) would be useful to avoid having to remember how to enable it otherwise, it reduces the bar to look for warnings.

On the other hand, a lot of times warnings will be caused by in-library code that the immediate user has no direct control over, hence we might not want warnings enabled by default. Warnings that definitely need to go to the user should be logged instead.