Fixed #16

Altered table mode behavior such that if there is a header line, i.e.
the first line of the table is separated by a border with the rest of
the table, then it is not taken into account for formulas and cell
arithmetic

t/fixtures/formula/formula.txt

@@ -4,4 +4,4 @@
 | Tomatoes |     5 |
 | Pasta    |   100 |
 | Total    |     0 |
-/* tmf: $5,2=Sum(1:-1) */
+/* tmf: $4,2=Sum(1:-1) */