These classes are subtly different, particularly in terms of their truthiness.

CmdResult
- attributes:  out, err, war
- truthy:  if err is falsy

CommandResult
- attributes:  stdout, stderr, data
- truthy:  if err is falsy AND data is not None

So CmdResult was oriented to provide essentially info, error, and warning messages to the user (typically as stirngs), whereas CommandResult is geared towards providing info and error messages to the user as strings in addition to data to the user in a command-specific format which is arbitrary other than it should never be None if the command succeeds.

Also:
- Refactored perror() to remove a rarely used optional argument which was unecessary

Updated argcomplete_bridge to use new constants/utils.

Added some shared definitions to cmd2/__init__.py -> maybe there's a better place for these?
Figured out how to trick bash into showing argument hints. It's a bit weird.
Updated all of the tests and examples to import cmd2 resources from their new location without the automatic imports in cmd2/__init__.py
For #369