Table footer with functions

[FedeClaudi]

Hey WIll,

I'm implementing a version of Tables in Term.jl in Julia, and I've just added a functionality that I thought could be useful in rich too.
The idea is pretty simple, when adding a footer users can pass a function, this gets applied to all values in each column and the result (with the function name) is shown in the footer. E.g. for a numerical table with footer=sum:

It's mostly useful for numerical tables only, but it makes it easier to use the footer to show column-wise means/sums etc...