Easy publish on CI env

[jaumard]

Hello :)

I've open this issue on pub https://github.com/dart-lang/pub/issues/2227

But look like it will take time to be implemented properly, I think it can be done in this plugin until an official solution is available.

A pubx publish --token that will set the token into /.pub-cache/credentials.json and then forward the publish to pub.

What do you think ?