Allowing grouped bar charts

[mdlemay]

Creating grouped bar charts like the example at https://vega.github.io/vega-lite/examples/bar_grouped.html requires that it be possible to encode the xOffset attribute, which does not currently seem to be covered in this library.

[ThomAub]

Is this part or the V5 specification only ?
If so I think #23 could be of some help :)

[mdlemay]

I do see x_offset show up in the current (v4) schema.rs, but I'm not positive whether that's the relevant one.