Emit COT alert when aircraft squawks emergency code

[chriselsen]

When an aircraft squawks an emergency code of 7500 (hijack),7600 (radio failure) or 7700 (technical, mechanical or medical) - which in the dump1090 JSON is indicate by the emergency value being set - emit a COT message for an alert. Possible COT alerts would be b-a for a general alert or b-a-o-tbl for a 911 alert.