Support MouseOver For Menu Instead Of Click

#### Feature Description
I would like an option to be available for MdMenu that will open it when the mouse is over the menu element, and close it when the mouse leaves the menu element, or the menu dropdown.

**Provide a brief summary of the feature you would like to see.**

**This is duplicate of the Issue #2080**. Our team encountered the same problem in our development and we had to use the hacky method @Marshal27 provided in https://github.com/angular/components/issues/2080#issuecomment-444488219.

It looks like alot of people in this thread legitimately wanted this feature to use, and there was even talks about how it would work, then just died out for some reason, with some later outrage from people about the ignorance of the issue.

I would like to reopen this Issue and once again discuss ways we can integrate the mouseover functionality into the material menu. I can certainly use it and it would benefit me and looks like alot of other people as well.

#### Use Case
**Describe the use case(s) that the proposed feature would enable.**

Use case would be in menu structures where a mouse over functionality would be favorable over button click. For us we are doing a notification icon that would display the type and number of notification on hover.

実装ファイルもテストも指定されていません。まず重複 issue #2080 と、そこで参照されている workaround コメントを確認し、次にメニュー要素と dropdown の両方について、ホバー時に開き、マウスが離れたときに閉じる想定の動作を明確にしてください。その動作がサポート対象の MdMenu オプションとして統合されれば完了です。

索引モデルが issue の本文から書いたものです。