Support npm userconfig / npm_config_userconfig

[timdp]

Do you want to request a feature or report a bug?

Feature.

What is the current behavior?

npm_config_userconfig is ignored.

What is the expected behavior?

npm lets you override the path to .npmrc through the config API. Yarn provides other ways of using alternative .npmrc paths but not userconfig.

[sebmck]

Created #1519 to fix this, pending approval.

[jblayneyXpanxion]

@sebmck does this work via command line in yarn?

Equivalent for npm:

npm install --userconfig ./.npmrc