Implement next_nearest_neighbors.

[leburgel]

It looks like we could make use of Graphs.neighborhood_dists to create a more general function for outputting the vertices that are the k-nearest neighbors of a specified vertex, for example:

function k_nearest_neighbors(g::AbstractGraph, v, d)
  nds = neighborhood_dists(g, v, d)
  # Filter `nds` for vertices with distances equal to `d`
end

Then next_nearest_neighbors(g, v) = k_nearest_neighbors(g, v, 2).

Originally posted by @mtfishman in #18 (comment)

[leburgel]

Was supposed to go in NamedGraphs.jl instead...