Allow calculations to return React component for the template

[AnsonT]

It'll be useful to support value editors if we can render the template value as a react component if it's a function instead of text.

[fairchild]

not quite the same, but similar, we added ability to use a component for a column header. In our case we needed a data filter. (It's on the list to send a pull request, after we add some tests.) It was pretty simple to add. If you want to see how we did it, as I think it is probably similar to what you are wanting to do, https://github.com/davidguttman/react-pivot/compare/master...FieldDayTech:master