Add a shell-completion-script generation feature

[natecook1000]

ArgumentParser should have the ability to generate code-completion scripts for different shells. This should initially support zsh and bash, for feature parity with the SwiftPM's completion-tool command, but be extensible to other shell completion styles as well.

One design would be to mimic the automatically-provided --help flag with a --generate-completion-tool option, that would take the name of the shell as its value. We'll need to augment ArgumentHelp so that an author can indicate the kinds of completions that are available, such as filename or path completion.

[natecook1000]

This was resolved by #123.