Added osmb.setFlatBuildingMaxHeight, osmb.enableFlatBuildingColors, and osmb.disableFlatBuildingColors

I find these functions useful for customizing how my building data is rendered. Enabling flat building colors is slower than leaving it disabled, but faster than drawing all buildings colored and 3D.

See this ticket: https://github.com/kekscom/osmbuildings/issues/32#issuecomment-16777197