support selection

v1 doesn't have [`.select()`](http://esri.github.io/cedar/api/select.html) nor [`.clearSelection()`](http://esri.github.io/cedar/api/clearselection.html) yet. This appears to effectively lock the hover color for marks that correspond to the given attribute value. For v1 we may need to also specify series?

Looking at the amCharts docs, I don't think they have the concept of a persisted "selection", so I think we'd have to persist it w/in cedar and apply the styles each time the chart is rendered.