Add new `skip` definition to skip templates when certain precondition is met

[clue]

It's a common use case to skip some templates if a certain precondition is met (such as "is this running in a Docker container?" etc.). As such, we should add a way to define a precondition in the form of a command line. This definition should be optional and confgen would only skip this template if this definition is executed successfully (prior to writing and files or executing any reload commands).