Add support for shell-completions file generation

[Coruscant11]

Closes #67

Hello @bradlarsen ! 👋
I worked on this very tiny issue for fun if you don't mind.
As you suggested in the issue, I used arg_complete.

A future improvement would be to generate these completion files at compile time, with the build.rs.
https://docs.rs/clap_complete/latest/clap_complete/generator/fn.generate_to.html

By installing Nosey Parker with an install script (package or cargo install for example), or even inside a Docker image, we could add auto-completion at install time.

Usage

noseyparker shell-completions --shell SHELL

If you quickly test with zsh for example :

noseyparker shell-completions --shell=zsh > noseyparker.zsh
source noseyparker.zsh

If you want to permanently add the completions to bash on Linux :

noseyparker shell-completions --shell=bash > noseyparker.bash
sudo mv noseyparker.bash /usr/share/bash-completion/completions/

Example

There is five possible values for the shell format :

➜  noseyparker git:(feat/clap_complete) ✗ noseyparker shell-completions --shell
error: a value is required for '--shell <SHELL>' but none was supplied
  [possible values: bash, zsh, fish, powershell, elvish]

For more information, try '--help'.

Here is the look when you toggle TAB for example for just noseyparker with zsh :

➜  noseyparker git:(feat/clap_complete) ✗ noseyparker
datastore          -- Manage datastores
github             -- Interact with GitHub
help               -- Print this message or the help of the given subcommand(s)
report             -- Report detailed scan findings
rules              -- Manage rules
scan               -- Scan content for secrets
shell-completions  -- Generate shell completions
summarize          -- Summarize scan findings

Or with shell-completions --shell :

➜  noseyparker git:(feat/clap_complete) ✗ noseyparker shell-completions --shell=
bash        elvish      fish        powershell  zsh

Completions seems to work for every commands.
I tested with zsh and bash.

---

Can you tell me if I should fix something ? 😄
Thanks !

[bradlarsen]

@Coruscant11 thanks for doing this! I will review this soon!

[bradlarsen]

This looks perfect! I'm just going to update the CHANGELOG to credit you, @Coruscant11 . Thank you!