[Sort] Add disable input

[Potmos]

Bug, feature request, or proposal:

Proposal: Allow the mat-sort-header tag to be attached to a boolean.

What is the expected behavior?

There should be a way to bind a column to a variable so you could enable or disable the the sorting.

What is the current behavior?

If there is the mat-sort-header tag on the column you can't disable the sorting.

What are the steps to reproduce?

https://plnkr.co/edit/SaM2RYwm3i7mRJhOIH2k?p=preview
I wish that clicking the button would disable the sorting.

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

This would make the sorting more flexible.

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

All of them

Is there anything else we should know?

[angular-automatic-lock-bot]

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