base_formatter: expose extra assets argument.
base_formatter: expose extra assets argument.

To let the users provide their own assets.

thiblahute/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,thiblahute/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,hotdoc/hotdoc,thiblahute/hotdoc,thiblahute/hotdoc