Add support for module references in the serialization

Uses the "%" symbol to reference a module, which is resolved relative
to the module/require of the serialization, and replaced by a module
descriptor as returned by Mr's getModuleDescriptor.

Example:

    // in ui/test.reel/test.html
    "module": {
        value: {"%": "./module"}
    }

    // results in
    {
        id: "ui/test.reel/module",
        require: <require function for package>
        // if the module has been loaded then other properties from
        // https://github.com/montagejs/mr/blob/master/docs/Module-API.md
        // are available
    }