Datepicker: make property "opened" an input

[ocombe]

Bug, feature request, or proposal:

Feature request

What is the expected behavior?

I would like to have the datepicker opened at load time, for that I expected to use [opened]=true, but it turns out that it's a simple property, not an input, which means that I cannot set the date picker to open at load time. If i use open() in ngAfterViewInit I get an ExpressionChangedAfterItHasBeenCheckedError, which makes sense since it was changed from false to true.

What is the current behavior?

opened is a property, not an input

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

We should be able to show the datepicker opened with an input

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

Material/CDK 2.0.0-beta.12

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