Improves /etc/apk/repositories format so you can say:
http://nl.alpinelinux.org/alpine/v2.3/main
@edge http://nl.alpinelinux.org/alpine/edge/main
@testing http://nl.alpinelinux.org/alpine/edge/testing
After which you can pin dependencies to these tags using:
apk add stableapp newapp@edge bleedingapp@testing
Apk will now by default only use the untagged repositories,
but adding a tag to specific dependency:
1. will prefer that tag for the name
2. allowing pulling in dependencies from that tag (though,
it prefers untagged packages to satisfy deps if possible)
fixes#575