add ability to set tooltips in parallel with data

This is some code that allows the user to set tooltips in google charts on multiple series as follows:

{name: {...}, date: {<some_date> => value}, tooltip: {<same_date> => value}}

Basically, it creates additional columns for each key to show the tooltip.  I'm not sure what happens if your tooltip doesn't have a corresponding date.

While I'm sure this is generally a great idea, I started doing this not because I wanted tooltips because I wanted to show the total value of a stacked area chart in a tooltip (see http://stackoverflow.com/questions/18699720/google-visualization-stacked-area-chart-add-total-to-tooltip).  This allowed me to create a tooltip series applying some extra code on each series of series I created and I then setting my options to chartkick as fllows:

      stacked: true,
      library: {
        focusTarget: 'category',
        series: {
          0 => {
            lineWidth: 0,
            pointSize: 0,
            visibleInLegend: false
          }
      }


P.S.  I would love to be able to add tests for this, but I don't want to impose a javascript testing framework on you.  Please let me know if there's one you prefer.  Thanks!