Nested packageProp path

Using cosmiconfig in a yarn version tool, yvm. It would be helpful to be able to use the yarn version in the package.json engines prop as a search place for configuration. e.g.

packageProp: 'engines.yarn',

A custom loader would probably work, but it seemed like a useful native feature regardless.

[iamogbz]

@davidtheclark I can put up a PR with this change, already have most of the code running locally.

[davidtheclark]

Yeah, I think this is something we should support, @iamogbz. Since this is the first request related to a custom package.json loader, I think I’m in favor of supporting dot notation (as @iamogbz suggests) instead of allowing people to specify custom package.json loaders — which seems like a more significant change, and I’m having a hard time thinking of what people might do other than the equivalent of dot notation.