Mapviewer's buildlayer doesn't support tooltip

Summary

tooltip enabled and applied via buildLayer doesn't apply

Steps to reproduce

• use g1.mapviewer and create a map via buildLayer -- map.buildLayer('layername', layer_opts) where map is a g1.mapviewer() object and layer_opts are layer options
• one of the attributes in layer_opts is tooltip: function(d) { return d.value; }

What is the current bug behavior?

Tooltip doesn't show up

Possible fixes

In src/mapviewer.js, do the below:

case 'topojson':
case 'geojson':
if() {
  ...
  self.renderTooltip(layerName, layerConfig)
  ...
} else {
  ...
  self.renderTooltip(layerName, layerConfig)
  ...
}

with this a developer can define

    tooltip: function(d) {
      // console.log(d)
      return d[_map_data['metric']] !== undefined ? d[_map_data['metric']].toString() : d[_map_data['metric']];
    },
    tooltipOptions: {
      direction: function (args) {
        return args.centerPoint[1] > args.tooltipPoint.x[1] ? 'top' : 'bottom'
      },
      sticky: true
    },

note that tooltipOptions is optional