format-table: add option to uppercase cells on display

format-table: add option to uppercase cells on display

This adds a per-cell option for uppercasing displayed strings.
Implicitly turn this on for the header row. The fact that we format the
table header in uppercase is a formatting thing after all, hence should
be applied by the formatter, i.e. the table display code.

Moreover, this provides us with the benefit that we can more nicely
reuse the specified table headers as JSON field names, like we already
do: json field names are usually not uppercase.