SIGTERM support for graceful shutdown

[0kate]

Description of the changes

1. Additional option to specify a signal type: I have added an option to specify signal types to killport cli. (like a traditional unix kill command)

• We can set -s/ --signal option to specify signal type.
• e.g. killport -s sigterm 8080

2. SIGTERM support: We can specify -s sigterm option for graceful shutdown.

Related issue(s)

This PR is related to the issue #8.
But this is not completely support for all signal types.

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

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

Checklist:

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

Additional information
Add any other information or screenshots about the pull request here.

[jkfran]

Thank you so much @0kate , I added a few small change requests. Let me know if you are okay with them.

[0kate]

@jkfran
Thank you for your review!
I have applied some changes about your change requests. Please confirm that again 🙏

[jkfran]

Thank you so much! @0kate