Improve pyscript documentation

[kmvanbrunt]

The section in Sphinx on pyscript command needs a bit of updating.
Topics to cover:

• Running commands with app()
• Developing a basic scripting API that only relies on stdout and stderr in CommandResult
• Developing an advanced scripting API that includes populating cmd2.last_result which becomes CommandResult.data
• Utilizing StdSim.pause_storage to avoid duplicating data already stored in CommandResult.data
• Processing a CommandResult object returned from app()
• Exposing custom data values with self.py_locals.

[tleonhardt]

Given the power of pyscript, our documentation has been sorely lacking in helping making this discoverable to end users

[kotfu]

We have some basic pyscript documentation now. I propose we move this issue out of the 1.0.0 milestone so that we can release. Any objections?

[tleonhardt]

@kotfu Sure moving this issue out of 1.0.0 sounds fine. We really could benefit from some improved pyscript documentation at some point, but it can happen after the 1.0 release.

@kmvanbrunt Do you have any pyscript documentation that you have been working on?

[anselor]

It's probably also worth nothing that the external test plugin simply opens up the pyscript as a cmd2 member function to make it easier to unit test cmd2 application commands.