Feature username argparse

[96ibman]

What this PR does

This PR adds a new option to check a specific, user-specified GitHub username instead of only generating random ones.

• Introduces --username <name> via argparse
• If the username exists, the script prints "USERNAME TAKEN" and shows the GitHub profile link
• If the username is available, it prints "USERNAME AVAILABLE" and writes it to available2usernames.txt

Example usage:

python checker.py --username 96ibman

Output (taken case):

USERNAME TAKEN: 96ibman
https://github.com/96ibman

Output (available case):

USERNAME AVAILABLE: somefreeuser

Other improvements

• Initialized colorama for consistent colored output across platforms
• Added timeout and User-Agent header for requests
• Preserved existing random scanning mode as default (non-breaking change)
• Added --sleep argument to control delay between random checks (default: 0.2s)

Why this matters

This makes the tool more practical for real users:

• You can quickly check if your desired handle is free without waiting for random scanning.
• Maintains backwards compatibility with the existing behavior.