Statistical footnote marks

[benediktclaus]

In statistical testing it is common to report p-values in a table and mark these with stars (*, **, ***) according to specific thresholds of p.

Thus, it would be great to have an option in opt_footnote_marks() that automatically assigns these footnotes to a given set of columns or at least be free in which marks to use.

[rich-iannone]

I agree that this would be a very nice to have!