Add preview mode customization

This hides certain commands as preview services.  A user
has to opt into this service via the config file if they want
to use this service.
Add _UNDOCUMENTED class attr to preview commands
Add test for provider help
Update README to catch casing changes
Fix typo
Merge branch 'preview-mode' into develop

* preview-mode:
  Add test for provider help
  Add _UNDOCUMENTED class attr to preview commands
  Add preview mode customization