lazy run modifiers

[jesteria]

The run modifiers supplied by argcmdr should vary from those of plumbum to permit lazy execution, and such that Local commands' output syntax needn't include tuples.

For example:

class PrintDir(Local):

    def prepare(self):
        return self.local.FG & self.local['ls']

The above is arguably clearer and more attractive syntactically than returning the tuple (self.local.FG, self.local['ls']).

In addition, the above syntax is closer (if not identical) to the use of run modifiers in plumbum.

(In the long run, the tuple syntax could also be deprecated, and eventually removed, simplifying that bit of argcmdr code.)