CloudFront invalidation of path when using S3 static website default index

Recently, a modification for CF invalidation was added so it was possible to invalidate files from a custom origin CF distribution that used S3 as custom origin.

Unfortunately, s3cmd wouldn't invalidate the path in CF when the uploaded file in S3 is the default index.

So, I modified it to automatically retrieve the default index name, detect if the invalidation path uses it and modify the query accordingly.

I also added two new flags to modify this behavior:
* --cf-invalidate-default-index: to force invalidation of default index file
* --cf-no-invalidate-default-index-root: to disable the invalidation of the default index file path