Create standalone "cash" executable that can launch concourse shell

[jtnelson]

cash --host <host>

There should be a cash script that can be installed independently of concourse server that allows connecting via the shell. The script should then be made available via homebrew brew install cash

Also worth think about a client side concourse CLI that can expose all the client side CLIs

concourse shell
concourse export
concourse import

Thought it would probably need a different name other than "concourse" so there are no name collisions.

[jtnelson]

The client concourse CLI might be too complicated for an MVP tbh