feat(tabs): align with 2018 material design spec

[crisbeto]

Aligns the tabs component with the latest Material design spec and adds the ability to align the tab labels in the middle or the end of the container.

[jelbourn]

LGTM

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[mmalerba]

@crisbeto I think we want to just not make the letter-spacing change here, it just doesn't look good in pretty much any of the apps with screenshot tests

[jelbourn]

+1 - I looked at the letter-spacing changes with Miles and they really just look awful across the board in real apps.

[crisbeto]

I've reverted the letter spacing changes.

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