Add caption

[ericnewkirk]

Adds caption and caption.color arguments to withSpinner. Captions are implemented as an additional div below the one containing the spinner. Not supported for spinner type 1 because the animation for that spinner causes the caption to move up and down with the animation, which is really goofy looking. Honestly I think 'if you need a caption pick any spinner other than number 1' is fair.

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