Shell completion

[WesCossick]

Support shell completion for programs built with Waterfall CLI. Those programs would probably need to register a postinstall script (reference) that runs a command like node entry.js initialize-completion, which Waterfall CLI would then catch and handle automatically. Likewise, a postuninstall script would be helpful too.

We'd need to consider other shells, too. Here's a package we could use, or potentially learn from: https://www.npmjs.com/package/omelette