Allow modular command definition

[anselor]

Allow commands to be split into multiple classes that are registered with cmd2 instead of requiring all commands to be defined in one giant class.

Breakdown:

• Define a base class for these satellite command definitions.
• Everywhere in cmd2 where it introspects for do_, complete_, or help_ also search in the satellite command objects.
• Stretch - auto-detect all satellite command definitions (by subclass) and load them automatically on init of cmd2.Cmd.
• Example of loading a satellite command object