Use CLI library to handle completion

Both github.com/urfave/cli and github.com/spf13/cobra have ways to automatically generate the bash completion information from our command structures.

- [urfave/cli v2 Bash completion](https://github.com/urfave/cli/blob/master/docs/v2/manual.md#bash-completion)
- [Cobra dynamic noun completion](https://github.com/spf13/cobra/blob/master/shell_completions.md#dynamic-completion-of-nouns)

The best long-term strategy is probably to switch to Cobra, and just use their functionality. Then users can get their completions just by running:
```
fscrypt completion bash
```

We would then be able to move completion logic into Go, and gain support for Bash, ZSH, and fish