feat(AppShell): shellRender and shellNoRender attributes and allow different element stripping strategies

[mgechev]

• Make the shell-related directives add shellRender and shellNoRender attributes.
• Encapsulate the "hiding", "showing" element logic into a service which is injected with DI and can its provider can be overridden.
• Add visitor which finds App Shell related comments in templates and processes them.

[mgechev]

Fixes #71.

[mgechev]

Note that in a browser environment, the DOM adapter needs to be explicitly set:

import { BrowserDomAdapter } from '@angular/platform-browser/src/browser/browser_adapter';
BrowserDomAdapter.makeCurrent();