Use CLI library to handle completion

[josephlr]

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
• Cobra dynamic noun completion

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

[ghost]

I am using Zsh. Please make completion in Zsh a reality!