Style overlay backdrop for specific dialogs

[YeomansIII]

Feature Request

What is the expected behavior?

Specify custom CSS classes for overlay backdrops for certain dialogs.

What is the current behavior?

Backdrops can not be individually styled. If I wanted to remove or style a backdrop it would have to be styled for all backdrops on all dialogs.

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

To remove/style backdrops for specific dialogs. One dialog may not need a backdrop, or a backdrop with a different color or opacity.

[YeomansIII]

I see OverlayState has the option of whether or not to include a backdrop.
https://github.com/angular/material2/blob/master/src/lib/dialog/dialog.ts#L143

Perhaps this option could be passed through the MdDialogConfig. That seems like a very easy change and I could make a pull request.

Still investigating how a custom CSS class could be applied to the backdrop.
This exists, but it looks it's been inactive for a few months. It also seems like they were making the change for a different purpose.

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