This allows to specify a table foot by enclosing it with part separator
lines, i.e., row separator lines consisting only of `+` and `=`
characters. E.g.:
+------+-------+
| Item | Price |
+======+=======+
| Eggs | 5£ |
+------+-------+
| Spam | 3£ |
+======+=======+
| Sum | 8£ |
+======+=======+
The last row, containing "Sum" and "8£", is the table foot.
Closes: #8257
