configurable decimal separator

Different regions and languages use different separators between the integer and decimal parts of a number, most commonly full stop or comma.

https://en.wikipedia.org/wiki/Decimal_separator#Usage_worldwide

As vim-table-mode bridges the gap between spreadsheet calculations and typesetting tables, it would make sense for its formula evaluation system to be able to work with different separators. At the moment, only fullstops appear to work.