[Feature] Check npm login and registry before publishing

[jamiebuilds]

If you aren't logged in to npm, lerna should stop and ask you to login. It would be good if it logged you back out afterwards.

[sebmck]

If you're doing this for security then it's pointless.

[jamiebuilds]

I have to switch between npm profiles for different (public and private) registries

[jamiebuilds]

I'm going to just do a check at the beginning to make sure that you're logged in to the right registry.

$ npm whoami
$ npm config get registry

Logging in is overkill, but I think the more checks we can do at the beginning is good.

[lock]

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