Definition supports legend

[tomwayson]

I suggest we base this mostly on the pro spec:

    "ChartLegend": {
        "type": "type: string; const-value: chartLegend",
        "visible": "type: boolean; description: true",
        "title": "type: string",
        "alignment": "type: string; domain: <left | right | top | bottom>",
        "valueFormat": "type: string"
    },

I say we start by supporting the ones that line up AmCharts legend properties:

Pro	AmCharts	Notes
visible	enabled
title	labelText	AmCharts also has accessibleLabel, should we set that?
alignment	position	in addition to the above values, AmCharts supports absolute, but we won't deal w/ that here, consumers will have to use overrides if they need that

A future iteration can support Pro's valueFormat by supplying AmCharts with a valueFunction, but I think we should be considering text formatting more broadly at that time.

For #375 the barest minimum that we need to support is legend: { visible: true }, but if title and alignment are easy to add, I'm not opposed to including those in the initial PR.

[tomwayson]

cc @phlorah @ajturner

[benstoltz]

Right now we have

export interface ILegend {
  enable?: boolean
}

Should be painless to switch that over to be visible and the other two parameters.