Allow for selection of rows (and columns?) from the top up with negative indices

```r
ht <- htable(mtcars)

# these would all work and do the same thing:
ht$row_bold(nrow(mtcars))
ht$row_bold(-1)

ht$row_bold(nrow(mtcars) - 1) # select second to last row
ht$row_bold(-2)
```

Should the same work for columns?