add mdTooltipClass to allow CSS customization of tooltips

[arlowhite]

Bug, feature request, or proposal:

The ability to specify a CSS class for a MdTooltip.

What is the expected behavior?

Developers should be able to customize some tooltips with CSS rules.

What is the current behavior?

Developers cannot apply CSS to a portion of tooltips because they are created in the cdk-overlay-container. Their only option is to apply CSS globally using .mat-tooltip.

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

A number of feature requests could probably be resolved if developers had mdTooltipClass
#3389

[devversion]

Developers would still need to disable the view encapsulation to reach to the tooltip overlay element.

Or is the main reason to be able to differentiate between different tooltips?

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