[prettybat] Add SVG and XML support

SVG may not be too hard to add, since it should be supported with ```prettier --parser=html``` already.

XML support has not been mainlined in ```prettier``` yet, so that would need a different formatter, like e.g. ```tidy```, but I haven't investigated enough to know what the best or most reasonable available XML formatter is currently. Not sure if that is something you would want to consider.