Permits exporting a string literal as a shim

I had a need for this so feel free to merge in if you think it might be useful to others. Basically just let's you shim a module as a simple string, i.e. in your config:

    "browserify-shim": {
      "./angular/angular-route.js": "'ngRoute'"
    }

So when required:

    var ngRoute = require("angular-route");    // ngRoute == 'ngRoute'

I specifically needed this so I could tie in seamlessly with Angular's module system:

    angular.module("ModuleName", [ require("angular-route") ]);

But yeah, might be a limited use case.