Support [md-menu-trigger-for] with anything conforming to MdMenu interface

[jelbourn]

Right now, [md-menu-trigger-for] requires that its value be an instance of MdMenu:

<button md-icon-button [md-menu-trigger-for]="menu"> Open Menu </button>

<md-menu #menu="mdMenu">
  <button md-menu-item> Hello! </button>
</md-menu>

It would be more flexible to allow the value to be anything that conforms to the expected interface (something like interface MdCustomMenu).

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