Add temporal, numerical x-axis + more styling

[jacobsorme]

Add numerical X-axis (opts.LineData{Value: []interface{}{1,2}}), including time.Date as X values. Add mark areas, mark lines and visual maps (charts.WithMarkAreaNameCoordItemOpts, charts.WithMarkLineNameXAxisItemOpts, charts.WithMarkLineNameCoordItemOpts, charts.WithVisualMapOpts).

[jacobsorme]

#44