npm installable nodes

Most of the code already exists for this, but there are some design decisions to be solidified before 'releasing' it.

On start-up, we search the node_modules path using the same scheme as require. This is done from the Node-RED install directory and down. We look for instances of package.json that contain a node-red property.

The node-red property contains a nodes property that identifies what nodes are provided by the module and where they should be loaded from.

    "node-red"     : {
        "nodes": {
            "foo": "foo/foo.js",
            "bar": "bar/bar.js"
        }
    }

Question: rather than just have the location is the value, should that be an object itself to enable additional properties to be defined in the future?