Allow `SilenceErrors` or `SilenceUsage` to override the rootCmd value.

[trajano]

I am trying to write something that augments the docker-cli so if it fails the parsing I would normally let it run the existing docker command. What I wanted to do though is add a new command that I know would not be in docker e.g. docker bash I would like to be able to make Cobra just show the usage or errors by setting SilenceErrors and SilenceUsage to true on the specific command