Yarn support

[brianalbers]

Meanio has a postinstall script with npm install hardcoded in package.json. If a parent application is built using yarn with meanio as a dependency, the meanio package dependencies are not loaded into the yarn cache. In turn this breaks the very powerful offline and cached dependency build features of yarn. The hardcoded npm install will not look at the yarn cache and the parent build will break.

Possibly allow an env var, some other external config option, to enable the postinstall script to use yarn instead of npm.

[timelf123]

Does yarn support githubuser/package#branch style versions yet? That was the only thing stopping me from messing around with it