format_number(X, D) - Formats the number X to a format like '#,###,###.##', rounded to D decimal places, and returns the result as a string. If D is 0, the result has no decimal point or fractional part. This is supposed to function like MySQL's FORMAT
Example:
  > SELECT format_number(12332.123456, 4) FROM src LIMIT 1;
  '12,332.1235'