[IMP] tags: introduce xml tag

Big change! This commit introduces an xml function tag to easily define
inline templates.

This is a pretty big change toward single file owl components

Part of #284

src/index.ts

@@ -10,6 +10,7 @@ import { QWeb } from "./qweb/index";
 import { ConnectedComponent } from "./store/connected_component";
 import { Store } from "./store/store";
 import * as _utils from "./utils";
+import * as _tags from "./tags";
 import { Link } from "./router/Link";
 import { RouteComponent } from "./router/RouteComponent";
 import { Router } from "./router/Router";
@@ -20,7 +21,7 @@ export const core = { EventBus, Observer };
 export const router = { Router, RouteComponent, Link };
 export const store = { Store, ConnectedComponent };
 export const utils = _utils;
-
+export const tags = _tags;
 export const __info__ = {};
 
 Object.defineProperty(__info__, "mode", {