Allow private npm packages as templates

Created by: itelo

Is your proposal related to a problem?

No. I just had to create a project like CRA in my work and we allowed private repositories. I want to know if it is a thing that the CRA community wants in the CLI. If it's ok I could work in a PR.

Describe the solution you'd like

probally a flag like:

npx create react-app --template some-template-name --npm-access-token="123"