Add support for column groups to the Table component

I’m proposing something similar to FixedDataTable (https://facebook.github.io/fixed-data-table/example-column-groups.html):

<Table>
  <ColumnGroup label="Name">
    <Column label="First Name" />
    <Column label="Last Name" />
  </ColumnGroup>
</Table>

The label prop can be either a string or a node. This is a use case in an application I’m building and I’ve worked around it by rendering an “empty” table above the primary table, but first-class support would be great. I can submit a PR, but would like to make sure this isn’t already in the pipeline.

[vakhtang]

@bvaughn Apologies for the delayed response. Indeed I’ve been very busy, but I have not abandoned this improvement. I’ll revisit it after the new year.

[MihirGH]

@bvaughn I want to use the Table component from the react-virtualized and have the same use-case of grouped columns. Is there any recommended way to achieve this?