v2.1.0 <zen-layout> release

Issue: #11

• Refactor the current combination of <mat-toolbar> and mat-sidenav-container and abstract it as a <zen-layout> component instead.
• Responsive mobile first design. Reference: Academind - Angular Material Responsive Navigation Tutorial
• Multiple ng-content projections entry points that can be selected via <div slot="..."> as named sections.

<!-- app.component.html -->
<zen-layout>
  <div slot="menu-content">
    ...Menu items...
  </div>

  <div slot="main-content">
    <router-outlet></router-outlet>
  </div>
</zen-layout>