Feature request: add color attribute to md-slider

[Mufasa]

Bug, feature request, or proposal:

It would be nice to have the option to set the slider bubble color to one of primary, accent or warn. Similar to the way we can do this for buttons.

What is the expected behavior?

e.g. should set the sliders bubble color to the themes primary color.

What is the current behavior?

I believe it currently always uses the themes accent color for the bubble.

What are the steps to reproduce?

N/A

Providing a Plunker (or similar) is the best way to get the team to see your issue.
N/A

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

My customer wants this change in color - so this is a customer driven requirement.

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

All

Is there anything else we should know?

No

[devversion]

I think this can depend on #2965 because I'm planning on creating a new directive for the color classes.

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