Handle rowspan/columnspan

Neither the speech rules nor the navigation rules work if `rowspan` or `columnspan` are given on an `mtable` element.

For navigation, it might be useful for the canonicalize step to mark row/col numbers and also `id`s of the up/down/before/after elements since those are hard to compute with xpath.

Navigation may also want to add a state variable for current row/col since moving to a spanned row/col potentially loses the current row/col when you move to a non-spanned cell.