add `line_graph` (vertex-edge-dual)

Hey, thank you for all your awesome work on graphs! : )

I have a request: could you add to operators the function line_graph?

Could be something as simple as:

function line_graph(g::SimpleGraph)
    edge_list = collect(edges(g))
    n_edges = length(edge_list)
    lg = SimpleGraph(n_edges)

    for i = 1:n_edges-1
        ei = edge_list[i]
        for j = i+1:n_edges
            ej = edge_list[j]
            if !isempty(intersect((src(ei), dst(ei)), (src(ej), dst(ej))))
                add_edge!(lg, i, j)
            end
        end
    end

    return lg
end

Start with the operators documentation entry point and the existing graph-operator APIs, then inspect how SimpleGraph, edges, and related behavior are tested. Use the proposed line_graph example as the starting specification; done means the vertex-edge-dual function is available under operators and its behavior is covered by tests and documentation.