Add support for a different default registry and for scoped registries

[sebinsua]

It would be good if this package had support for a different default registry and additional scoped registries.

This would help users in an 'Enterprise' setting. Especially if their private packages do not exist on registry.npmjs.org at all.

[orta]

Sure, yeah, makes sense - I don't have anything like this to test it on, so I'm afraid this one is on you 👍

[glensc]

For the test you could use registry.yarnpkg.com, it's just a proxy:

• What is the role of 'registry.yarnpkg.com'?  yarnpkg/yarn#889 (comment)