Support table formatting as one unit

[foster999]

Is your feature request related to a problem? Please describe.
I'd like to be able to overline and underline the whole table to make it look pretty. This would include the data, indexes and column headings.

Describe the solution you'd like
An additional element in the Theme object, that can set table-wide formatting.
This should come between global and single attributes in terms of precedence.

[foster999]

This type of formatting is currently supported by GPTable(..., additional_formatting = ...), but creating a theme element for this approach would allow it to be applied consistently across tables