Add `wordtree` chart type

[pchhibber]

My team has been working with polymer for a lot of projects, and we're considering using the chart component.

Could you please add support for word trees.

As per the current documentation, only the following types are supported.

area, bar, bubble, candlestick, column, combo, geo, histogram, line, pie, scatter, stepped-area, treemap

Thanks.

[wesalvaro]

package: wordtree
google.visualization.WordTree