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).

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}