Export Fabric

We're using a custom build of Fabric.js internally, for both
`node-markup`, and the main app/UI. To make dependency management
easier, lets export our Fabric dependency from the builder. This lets us
avoid manually managing the peer dependency between the two.

ImageMarkupBuilder.js

@@ -3,6 +3,7 @@ const Shapes = require("./shapes");
 const path = require("path");
 
 const Fabric = require("fabric").fabric;
+module.exports.Fabric = Fabric;
 
 /**
  * Expects a Fabric.js Canvas