Feature request: Set height and width based on number of items

[fbreitwieser]

Currently d3heatmap requires a height and width argument in d3heatmapOutput. When having varying input sizes, it is difficult to find values in which the line heights are the same, especially when displaying dendrogram.

It would be great if we could set a line height or column width, and the absolute size of the plot is changing based on that.