cmds2: commands
cmd2: simplified main

Attention @maybebtc @mappum

I cleaned up + simplified the main flow. Now, all printing
is contained inside main itself! (:cheer:). I do this with
the help of a cmdInvocation struct that has both
a Parse and Run. The only major clunkiness left is that the
"CallCommand" is still its own function. But *shrug*.

Please test it works as we would expect. i changed much of
the flow, so likely that i missed a complicated edge case.

main roadmap:
- parse the commandline to get a cmdInvocation
- if user requests, help, print it and exit.
- run the command invocation
- output the response
- if anything fails, print error, maybe with help
cmd2 fixed config panic

The way the current marshallers marshal out output requires
a ton of error checking. I wish there was a way to have the
library call our marshaller with the right type (rather
than an interface). Maybe can do this with Reflect someday.