insertRow inserts a new row in the table.
var row = table.insertRow(index)
row is a reference to the new row.
indexis the row index of the new row.
index is -1 or equal to the number of rows, the row is appended as the last row. If
index is omitted or greater than the number of rows, an error will result.
newrow = mytable.insertRow(0); // insert a new first row