proposal : datepicker add popup position

[ghost]

Bug, feature request, or proposal:

proposal

What is the expected behavior?

if the datepicker in the bottom of a page, the calendar popup above the input

What is the current behavior?

only under the input ,when the touch ui is false

What are the steps to reproduce?

https://material.angular.io/components/component/datepicker go to examples and click the calendar button

What is the use-case or motivation for changing an existing behavior?

to the purpose that i can select date,if the datepicker input is put in the bottom of page,or there is no enough space to display the calendar under the input in the case of the page is scroll to the bottom of a page.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material 2.0.0-beta.5
Google Chrome 58

Is there anything else we should know?

[donroyco]

This has already been mentioned in #4406

[jelbourn]

Dupe of #4406

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