feat(datepicker): add theming support

[crisbeto]

Adds support for theming the datepicker using the color attribute, as well as inheriting the theme color from the form field around a datepicker input.

[jelbourn]

@crisbeto@mmalerba why is this major instead of minor release?

[mmalerba]

@crisbeto needs rebase

[crisbeto]

Rebased.

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