Allow to configure the background color when generating SVG

[ggrossetie]

I think it would be useful to be able to configure the background color when generating SVG. One way to do that, is to introduce an option background-fill with a default value transparent (backward compatible).

If this option is set, BlockDiag could add a rectangle with a width and height of 100%:

<rect xmlns="http://www.w3.org/2000/svg" width="100%" height="100%" fill="{background-fill}"/>

Here's an example when using background-fill: #007c92: