Add CSV output option

[dennis-tra]

If the --neighbors and, e.g., the --csv flag is given a neighbors adjacency list should be generated after the crawl. Format:

peer_id_1,neighbor_1
peer_id_1,neighbor_2
peer_id_1,neighbor_3
peer_id_1,neighbor_4
peer_id_1,neighbor_5
...
peer_id_2,neighbor_x
peer_id_2,neighbor_y
peer_id_2,neighbor_z
...