Add branch selection option to rendergit #25

Description:
Currently, rendergit always clones the default branch (main/master) of a repository. It would be very useful to allow users to specify which branch to render.

Proposed solution:

• Add a CLI flag (e.g. --branch <branch_name>) to rendergit.
• When provided, rendergit should clone the specified branch instead of the default one.
• If no branch is specified, fallback to the default branch as it works today.

Example usage:

rendergit https://github.com/karpathy/nanoGPT --branch develop