Support for remaining signals

[NachiketNamjoshi]

Description of the changes

• Adds support to send all signals available in nix::sys::signal::Signal and works with the same -s option.
• Refactored integration_test.rs

Related issue(s) Improves on #8

Type of change Please select one or multiple of the following options:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [X] Code cleanup (refactoring or improving code quality)
• [ ] Documentation update (adding or updating documentation, updating README)

Checklist:

• [X] I have read the CONTRIBUTING document.
• [X] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.
• [X] I have added tests to cover my changes.
• [X] All new and existing tests passed.

[jkfran]

Thank you! Could you format the code with cargo fmt --all?

[NachiketNamjoshi]

I ran cargo fmt --all. Please, verify.