Allow a raw ARGV array to follow the options.

This is for the case where users want to take advantage of the ENTRYPOINT
pragma, as requested in #31
Adding missing itertools import
Adding some documentation for slash-args
Draft solution to #30
Fixes from thread on #30
Propagate index settings (from #30).
Tolerate unsused arguments in image_token()

For #30
Fixing error messages when using dockercfg setting