"npm tag" is unsupported

[rlidwka]

...

[rlidwka]

Proposal:

1. dist-tags is a hash "tag -> array of versions", not "tag -> version"
2. when we are retrieving something, return highest version having this tag
3. once you tag something, there is no way to remove that tag except for unpublishing
4. latest tag always returns highest available version, because current magic is just stupid

This would allow something like that:

npm tag something@1.2.3 production
# "dist-tags":{"production":['1.2.3']}
npm install something@production # installs 1.2.3
npm tag something@1.2.4 production
# "dist-tags":{"production":['1.2.3','1.2.4']}
npm install something@production # installs 1.2.4
npm unpublish something@1.2.4
# "dist-tags":{"production":['1.2.3']}
npm install something@production # installs 1.2.3