Consider expanding scripting capability to include conditional control flow

[tleonhardt]

The current built-in scripting capability is very nice for automating the execution of a series of commands.

However, there isn't any built-in way to run a command IF certain criteria are met (based on say whether or not a previous command "succeeded".

This may be beyond the scope of what cmd2 is intended to provide. But if there was an easy way to add this functionality, it would be useful.

[tleonhardt]

Adding this capability to the the simple ASCII-text scripts invoked via the load command or @ shortcut would be very difficult.

However, we could make it easier for users to add conditional control flow to python scripts which can be run with the py command.

The main thing we would have to do is to add a standard way of storing off "results" from the last command and making it easy for users to get to them from a script. I'm tentatively thinking of creating a Result namedtuple for this purpose.

[tleonhardt]

My idea is to add something like this:

from collections import namedtuple

#noinspection PyClassHasNoInit
class CmdResult(namedtuple('CmdResult', ['out', 'err', 'war'])):
    """Derive a class to store results from a named tuple so we can tweak dunder
    methods for convenience."""
    def __bool__(self):
        """If err is an empty string, treat the result as a success; otherwise treat it as 
        a failure."""
        return not self.err

Then we can add an attribute called something like self._last_result, which can be used within Python scripts. Users would have to make sure that the commands they care about scripting save their results to that self._last_result attribute.