Add argument to login to Instaloader

[RyanLua]

Description

Adds a new argument to start an interactive login with Instaloader.

Related Issues

Closes #16

Changes Made

• Add new --login argument which takes a username
• Update previous arguments to use argparse metavar when needed
• Add logic for logging in using Instaloader with interactive_login()

Checklist

• I have tested these changes thoroughly.
• I have reviewed my code for any potential errors or issues.
• I have followed the code style guidelines for this project.

Additional Notes

Related to #15

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

OpenSSF Scorecard

Package	Version	Score	Details

Scanned Files

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/instawebhooks/parser.py:52

• [nitpick] The help message could be more descriptive. Suggest updating it to: 'login to Instagram with username, starting an interactive login session.'

help="login to instagram with username"

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/instawebhooks/main.py:58

• The error message is vague. Consider specifying the type of error that occurred during login.

An error happened during login. Check if the provided username exists.

src/instawebhooks/main.py:48

• The new login functionality should be covered by tests to ensure it works as expected.

if args.login or args.interactive_login: