Add tab-completion helpers for positional and flag-based completion

[tleonhardt]

Add helper functions in cmd2 to assist with tab-completion in the following scenarios:

• Completion following an argparse flag
• Completion based on argument index/position

Include an example of how to use these and unit tests.

[tleonhardt]

@kmvanbrunt You should possibly be aware of issues #274 and #277 - they may have some degree of relevance towards what you are working on for this issue.