Feature request crane: support using 'https://' and "http://" in registry URI

[johnpoth]

Great library !

Right now there is a bit of magic going on when crane tries to automatically decide whether to use the http instead of https. Fortunately there are some tricks to make crane use https when running the registry server locally. That workaround is a bit tedious to set up and it's not clear to the user at first sight why crane decided to use http when the --insecure wasn't specified.

A more user friendly approach would be to allow prefixing image URIs with_http://_ and https://.

I would be happy to provide a PR for this.

I would also remove the magic and default to using https:// unless the --insecure flag is specified. This seems similar to what curl does.

As a bonus feature, i'd allow passing certificates via the command line (curl style) but that's another issue :)