ui.table format default values

[mattrunyon]

There are cases where a user may want to reset a formatting value to default with a higher priority rule.

For example, in this code the 2nd rule would apply background_color and the user may want to switch back to our default color handling which would pick a contrasting light/dark color instead of applying the color from the first rule.

format_=[
  ui.TableFormat(color="negative"),
  ui.TableFormat(cols=["X", "Y"], background_color="negative",  color="default")
]

We could handle this as the difference between None and Undefined with #549. However, it might make more sense semantically to allow a value like default