Implement table footer in BasicTable and maybe MemoryTable

[timroes]

It would be nice to have a possibility for footers in tables (we would require that for table vis).

I see footers be possible in two different ways:

• Define a column specific footer (e.g. via the column definition). That will be rendered as the footer below that column.
• Often you might want to have an overall footer, that might span several columns, so I think it would be nice to have also the possibility to specify a table footer, that can use colspan.

Why are these needed?

• They should properly render tfoot for better accessibility and semantics.
• The footer needs different (font) styling, value formatting (i.e. not use the cell render function).
• Not take part in any sorting or counting for pagination.

[lukeelmers]

I can take a look at this as it's a prerequisite for elastic/kibana#16639

[snide]

@lukeelmers if you take this on, just pass a PR over to myself, @cchaos or @ryankeairns and we'll clean up any styling it needs. It's OK to pass it over functional without styling.

[lukeelmers]

@snide Awesome -- will do!