Add shell tab completion

[jonmast]

Clap supports this, so shouldn't be too difficult to add basic support. Ideally we'd extend this to have context aware completions for app names and the like.

We'd also need to figure out the install story for this, not sure if we want to bundle it with our releases or not. Brew has something built-in which should cover that use-case, it may not be necessary for the standalone builds.