add a shared `dimension` prop and make `dimensionConstructor` optional

[Spacarar]

some of the existing charts in CC share a dimension which allows them to be unaffected visually by the filters of the other charts.

Currently vue-dcjs doesn't support this out of the box, but it's really close to being able to.

because all charts store their dimension in this.$options.dimension already:

• this.dimension is the prop for a shared dimension
• this.dimensionConstructor is a prop function for an enclosed dimension
• this.$options.dimension is either this.dimension or the result of this.ndx.dimension(this.dimensionConstructor)