Allow iModel.js to extend the design review app

Allow iModel.js to extend the design review app Bentley is maintaining

The Imodel.JS is available with only a light weight viewer that is lacking most features of the design review app. This means we will have to rebuild large portions of the tool. I understand the design review app is considered propitiatory but it seems like the real value in iModel.js would be in allowing reuse of what you already have so we can extend its capabilities for our data.