Usage improvements for S3 bucket weblinks generation

[daviddays]

This PR includes several small improvements to help the typical user run the script against their own S3 buckets without having to modify the existing python:

• Addition of main-method check and runtime argument parsing to pass in buckets, templates, etc as parameters
• Modification of index link generation to add entries for parent directory (if available)
  

[ryfeus]

LGTM! Thank you for improvements!