jxc properties in package.json

[karaxuna]

Usage:

{
    "jxc": {
        "plugins": ["cordova-plugin-camera"],
        "platforms": ["android", "ios"]
    }
}

Note that jxc properties are read from sample's package json, not from <sample>/www/jxcore/package.json

[ktrzeciaknubisa]

It took some time to check this, but it is a nice work. Thanks!