Add Interface

**I'm submitting a feature request**

* **Library Version:**
Latest
major.minor.patch-pre

**Please tell us about your environment:**
* **Operating System:**
Win10

* **Node Version:**
6.10.2

* **NPM Version:**
3.10.10

* **Aurelia CLI OR JSPM OR Webpack AND Version**
CLI 0.27.0 | JSPM 0.16.53 | webpack 2.3.3

* **Browser:**
all | Chrome XX | Firefox XX | Edge XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView

* **Language:**
ESNext

**Current behavior:**
Hi
Recently I work on a map base project with dojo and I realize it has interface for its widget(module) input
I think that will be great if Aurelia add it too so in top of my module I can say in code what object I expect from user and if user use Aurelia helper in vs code and tools like that it can show suggestion for him and make it easier to use a module without doc it can use in output too on service and even help to implement factory design pattern in future. I'm using. Net core in server side but interface can really help in Client site especially when you design plugin and let your backend programmer create simple form with them

The issue names no files, tests, or implementation entry points; start by reading the Aurelia framework entry point and the eight-comment discussion to determine whether the request concerns TypeScript interfaces or runtime behavior. Done is an agreed scope with identified implementation and validation points, which this issue does not yet provide.

Written by the indexing model from the issue text.