Add new feature: yarn background scripts [WIP]

[magicmark]

Here's a stab at a crazy idea: yarn background scripts.

e.g.

"backgroundScripts": {
    "express-server": "nodemon lib/index.js",
    "babel-compiler": "yarn build --watch",
    "mock-json-server": "json-server --watch db.json"
}

enables:

It's not unlikely that we decide this feature doesn't belong in yarn, but I think there could be some benefits to doing so, as detailed in the RFC.

This is an early stage RFC - at this stage, trying to gauge at a high level if this feature belongs in yarn or not.

Thanks!