cod should emit standard completion scripts

[zachriggle]

It would be useful to have cod be able to generate either bash or zsh completion scripts, so that they can be distributed with a project instead of suggesting that users use cod.

[dim-an]

Though I understand why this might be useful I don't think that I want this in the cod in near future.

Such approach reduces reliability and maintainability of the cod and features like #4 would be harder to implement and maintain.