Rename and improve --interface-service

[johnsca]

Rename the --interface-service option to --layer-index to better match what it is. Also, we should accept normal GitHub URLs, including forks and branches (e.g., https://github.com/Cynerva/layer-index/tree/gkk/test-subdir), and automatically convert them to the appropriate raw.githubusercontent.com form. Also, be more lenient about trailing slash or not.