plot: Can't add data labels to the barchart

### What are you trying to do?
I am trying to add data labels(show the value of each bar on top of it) to a bar chart.

### What did you expect to happen?
A field in the [BarChart](https://godoc.org/gonum.org/v1/plot/plotter#BarChart) struct which can set it to show the data labels or not and even where to put them.

### What actually happened?
Right now there is no field in the [BarChart struct ](https://godoc.org/gonum.org/v1/plot/plotter#BarChart)

### What version of Go and Gonum/plot are you using?
Go: 1.10.3
Gonum: [5f3c436](https://github.com/gonum/plot/commit/5f3c436ce602d80b13f2936b6bf149515198d6a4)