Support hidden commands

[jvalkeal]

Sometimes it'd be nice to have a commands which are either experimental or something which are in place but not exposed to user either via command completion, shell completion scripts, etc. You just want command to execute if user knows it's there and what are its options.

One use case in a spring-shell itself would be a sample application which we use for e2e testing. We'd have a lot of root level commands starting with e2e and would not like to expose those to sample app itself for help or completion.