[FAILING] Test for embedding <Td>s in <Tr>s
Make it possible to include <Td>s as children

Make it possible to include Reactable.Td as a child of Reactable.Tr,
and specify a `column` property to the Tr to control the order that it
renders in, and a `data` property to override the default of grabbing
the data for that cell from the child string.
Add documentation for `Reactable.Td`