Feature Request: sub-cells by direction.

First of all, I'd like to say that it's really a pleasure to use this library.

I am having a use case for which I need to identify "border cells" below a certain distance, e.g. all cells with resolution of about 2.4x2.4 m^2 (geohash lenght = 9) on the border between cells "b" and "c".

What I would require in this case is, being able to extract all cells of geohash length n+1 (e.g. lenght 2 starting from cells of length 1) by direction, so to have the rightmost column of sub-cells in "b" of geohash length, say, 9, and the leftmost column of subcells of "c" of the same geohash length.