jshell tool: /retain all -- and other refinements

Add:
   /retain all
to retain all settings.
Add:
   /retain all -default
to reset editor, start, and feedback.
Add:
   /retain all -default -delete
to also delete all retained modes and plugins.

Also, add undoing fluff:
  jshell> /retain editor teco
  Editor set to teco
  To return to default editor use: /retain editor -default