Switch command parsing from cmd.Cmd to argparse.ArgumentParser

cmd.Cmd is focused more towards a REPL (with completion etc) than taking args from the command line. And doing it this way means we aren’t trying to parse cli arguments manually.
Rename src/shaker/shaker.py to avoid confusion.

This file solely deals with parsing cli args and then invoking shaker, so lets say that.
Add in back-compat shim for existing cli argumaent form

This is currently used in this form via .kitchen.yaml (for instance <https://github.com/ministryofjustice/metrics-formula/blob/d87a59f8b7e86378e0a310c8b47e5393e131d3d2/.kitchen.yml#L25>) and having this shim makes the upgrade path a lot easier.
Merge pull request #5 from ministryofjustice/cli-parsing

Switch to argparser for parsing args