Accept an array as `valuePath`

I often find myself needing to compute a cell's value dependent of more than one field of the row. But the current API allows only passing one key as the `valuePath`.

I work around this by just accessing whatever properties I need from the `row` attribute that is passed to the `cellComponent`. I think that's pretty ugly.

If we would allow users to pass a string or an array of strings to `valuePath`, then (in the latter case) we could pass an array of values as `value` to the `cellComponent`.