Enhancement: Add pretty print support for output

[mwales]

Along with the output routines like poutput, pwarning, etc you could add a pretty print output. The alternative is the caller has to use pprint.pformat to pretty print the data before sending it to cmd2's output routines.

[tleonhardt]

@mwales I like this idea and it sounds like an easy addition and a good first-PR for someone.

@kmvanbrunt Do you have any objections to adding this feature?

If there are no objections, @mwales would you like to submit a Pull Request?