Expose aggregator outlet to allow rendering additional content in aggregatorCell

[nselikoff]

This PR introduces an additional key on aggregators, outlet, a function which, if present, gets called once by PivotTableUI to render additional aggregator-specific content just below the built-in aggregator selector (in the area outlined in red here):

Some use cases for providing this escape hatch include:

• rendering aggregator-specific custom UI controls
• rendering aggregate stats that you want visible regardless of the state of the pivottable attributes

When the outlet function is called, it will be passed the PivotTableUI's props.data, in case the called aggregator wants to make use of that data.

Here's a kind of silly example of a custom aggregator using this new function:

  function customCount(formatter) {
    return () => 
      function() {
        return {
          count: 0,
          push() {
            this.count++;
          },
          value() {
            return this.count;
          },
          format: formatter,
          outlet: (records) => {
            return (
              <div>There are ${records.length} records total</div>
            );
          },
        };
      };
    };
  },

Resolves #117

[nselikoff]

I split out the build from the doPublish so I could include my local branch of react-pivottable in another app to prototype the new feature.

[nicolaskruchten]

Seems reasonable to me! I don't have time just now to test and release but I will do so within a couple of days. Please ping me again if I don't get to it?

[nselikoff]

For sure, thanks!

[nselikoff]

Hey @nicolaskruchten pinging you on this :)

[nicolaskruchten]

Sorry for the delay... this is now out in v0.11.0.

If you want to keep working on this, maybe I can add you as a contributor and let you do releases?

[nselikoff]

Thank you very much for this and apologies for the long delay in responding. Thanks for the contributor offer as well, but I need to pass for now.