permit negative indexes on row_spec and column_spec

[mgacc0]

Would you consider of interest implementing the possibility of specifying negative indexes on the row number of row_spec (and also on the column number of column_spec)?
It is very common needing to apply a certain format to the last row. And it would be easier if we could write row_spec(-1, ...).

[dmurdoch]

In R, an index of -1 generally means every entry except number one, so I'd expect that to mean all rows except the first.

[krivit]

Up until 989d6dc, row_spec(-1, ...) would affect the first row of a two-row header. However, this does not appear to have been intended functionality but rather a serendipitous feature.

Since 989d6dc, it just produces an error.

[dmurdoch]

I think the proposal in #827 makes more sense in R than this one, so I'm going to close this issue.