polyfill missing stuff for IE

[gordonwoodhull]

dc.js 3.* still supports IE, sad as it is.

D3v5 introduced a couple of ES6 library dependencies, Fetch and Promise. We should include polyfills on all our pages so that they still work in IE.

[gordonwoodhull]

They are easy to polyfill; just grab whatwg-fetch and promise-polyfill from NPM, and include the libraries in your page before the use of d3.fetch.

I tried this on the main page and IE works again. I'm now looking into how to conveniently add it to all the pages (or at least those that use Fetch). I guess none of our tests use d3.csv or d3.json or we would have noticed this earlier.

Thanks @sgavathe for the report!

[sgavathe]

@gordonwoodhull Thanks for giving a solution to it! The polyfill fixes for IE does makes sense.

[gordonwoodhull]

Fixed all examples in dc.js; will now proceed to dc-js/dc.datatables.js#7

[gordonwoodhull]

There are a few remaining arrow functions and Object.assign - following up with a few more commits here.

[gordonwoodhull]

boxplot and sunburst examples now work - the Object.assign polyfill is built-in and the Array.find polyfill is pasted into the relevant examples. All examples are tested to work.