support for custom formatters in chart object

With this change you can add field `customFormatters` to the chart object. 
It's a map from formatter name to its class.

Such formatters still need their options to be set in `formatters` section(under corresponding name).

Example configuration(coffeescript) 

    $scope.chartSpace =
      type: "PieChart"
      options:
        title: gettext "Space distribution"
        legend: "none"
        pieSliceText: 'value'
      customFormatters:
        size: SizeFormatter
      formatters:
        size: [
          columnNum: 1
        ]
      ...
      
where `SizeFormatter` is a formatter class as per Google Charts' api.