added support for bash native flags e,o,x (#44)

* added support for bash native flags e,o,x

* restore values when script finishes or after error

* restore values at any exit case

Fix calling with only flag argument yielidng "Empty argument" (#36)

Hi there,
Doing something like `example.sh -hp` before this PR would yield 
```
[ERROR] Empty argument "happy"

Usage: <...>
```
This small addition should fix this behavior. I don't know this script all that well, but I ran the tests and they all seemed to pass. I would still like to ask for a review of this, as I'm not sure if I'm breaking crucial program logic here and just not seeing it 😅 

Thanks for creating this awesome script :)

Cheers!

Feature/bash shebang (#31)

* use recommended shebang

* easy copy of code snippets

* updated alpine version

fixed getting the exact env var name

support ctrl+c

Updated by GitHub Actions

Updated by GitHub Actions

use env var only if not empty

Updated by GitHub Actions

Merge pull request #17 from unfor19/add-shellcheck

Add shellcheck