feat: implement support for npmrc options set to environment variables

[alienriver49]

This adds support for a .npmrc file similar to the following:

registry="private artifactory registry url goes here"
_auth=${ARTIFACTORY_TOKEN}
email=${ARTIFACTORY_USERNAME}
always-auth=true

[guybedford]

Thanks for getting around to it! Published a 1.0.2 let me know if that works out ok there.

If you hit other things let me know - may be able to get you publish access.

[alienriver49]

Thank you! Would you be cool if we cherry-picked this commit into the 0.17 branch and released a new version of that as well? If you are cool with that, want me to open a PR for that?

[guybedford]

@alienriver49 sure we can do an update for 0.17 as well no problem.