Add `filter` property to `cross` transform
Add `filter` property to `cross` transform

Implements/resolves vega/vega#338
linearregression/vega,nyurik/vega,vega/vega,cesine/vega,mathisonian/vega-browserify,cesine/vega,vega/vega,mcanthony/vega,mcanthony/vega,carabina/vega,linearregression/vega,carabina/vega,timelyportfolio/vega,uwdata/vega,timelyportfolio/vega,vega/vega,smartpcr/vega,seyfert/vega,vega/vega,nyurik/vega,cesine/vega,lgrammel/vega,nyurik/vega,uwdata/vega,seyfert/vega,smartpcr/vega,linearregression/vega,mcanthony/vega,mathisonian/vega-browserify,mathisonian/vega-browserify,smartpcr/vega,carabina/vega,seyfert/vega