grouping and folding feature

[jjagielka]

Grouping (subtotals) of data by dimension and the possibility to fold/unfold groups.

Minimal changes in:

• Utilities.js (PivotData class) - grouping in processRecord function and some tweaks to sorting
• TableRenderer.jsx - display in groups, folding capability (~10% of code changed in TableRenderer.render function)
• App.jsx - grouping props exposed as checkboxes

Demo: https://jjagielka.github.io/react-pivottable-demo/

[newt0311]

I was about to (finally!) look at my PR for subtotals again but then I saw this. It's great!

@nicolaskruchten any preference between this PR and mine? I would like to get subtotals into react-pivottable but not picky on how...

Thanks.

[baur]

Hi, @nicolaskruchten Please, merge this cool PR!!!