json output format for address list

[splix]

Introduce a new option --format=<text|json>

By default the output format is text, for a human. Another alternative would be JSON output that simplifies integration with external tools.

emerald --chain=morden list
ADDRESS                                       NAME
0x907c6c1a17707968fd238b91263fde5a076b20c3
0x60da613e63a6180a565a0dedcebb2770fbb46946    test 2

emerald --chain=morden --format=json list
[{"address":"0x907c6c1a17707968fd238b91263fde5a076b20c3", "name": null}, {"address": "0x60da613e63a6180a565a0dedcebb2770fbb46946", "name": "test 2"}]