This patch adds the notify plugin to be used with the new
notification plugins interface for aprsd. This sends a notification
to a slack channel when a HAM callsign is seen on the APRS-IS network
after a specified timeout/age.
Unfortunately, this currently only works in the US. need a global
reverse geolocation service that provides an area description
"6 miles northeast of X"
This patch ensures we check to make sure the required config
keys are available from the aprsd.yml config prior to accessing them.
The plugin won't work without them. This prevents the plugin from
throwing stack dumps in aprsd.
