add a default trigger that triggers for prolonged silence about loudometer

[saucecode]

Attempted implementation in v1.0.0. 0fb17df
Configuration file entry now looks like:

"default_trigger": {
    "http_target": "http://127.0.0.1:8888/press/bank/5/3",
    "channels": [
        0,
        2,
        3,
        4
    ],
    "silence_level": 5,
    "silence_time_ms": 5000,
    "delay_ms": 500,
    "trigger_hold_time_ms": 1000
}

i.e.
If the channels 0, 2, 3, and 4 have an accumulated volume of less than silence_level 5 for longer than silence_time_ms (5 seconds), this trigger will arm.

This default trigger has priority of -1000, so should be disarmed by the arming of any other trigger in the configuration file.