Add support for other shells

[Yusyuriv]

Why

I really liked the idea of the tool, but unfortunately I'm using Windows. The tool installed just fine, but it had no support for PowerShell.

Changes

By default, it keeps working just like before, this will keep working:

export $(envelope list dev)

I added a new argument, --shell, that changes the format of the output. It's optional, and by default it's set to kv (key-value pairs, just like before). In addition to that, it accepts some other values:

• sh/bash/zsh:

  export API_KEY='sk_...'
  export ENV='dev'

  I did this because the original list of key-value pairs can fail if the values have spaces or some special characters in them.
• nu/nushell:

  {"API_KEY": "sk_...", "ENV": "dev"}

• cmd:

  set "API_KEY=sk_..."
  set "ENV=dev"

• powershell/pwsh:

  $env:API_KEY = "sk_..."
  $env:ENV = "dev"

Docs

I updated man/envelope.1.md and README.md, feel free to let me know if I missed something or if you'd like to have something rephrased.

Tests

I added tests that verify outputs for each shell, including testing for correctly escaped special characters in each shell.