Nagios: Adjust messages posted to slack to allow keyword alerting

In order to get good notifications for important outages, it would be beneficial for certain classes of messages to be listed with a particular keyword that you can trigger slack notifications on.

I tried setting my notifications to alert me the word "CRITICAL" which will catch things like this:

`HOST: test-osuosl-ubuntu1604-ppc64le-2 SERVICE: Current Load STATE: CRITICAL MESSAGE: CRITICAL - load average: 20.84, 20.96, 21.21`

But not these:
`:fire: HOST: test-marist-ubuntu1804-s390x-2 SERVICE: STATE: MESSAGE:`

`HOST: build-equinix-centos8-armv8-1 SERVICE: RAM STATE: UNKNOWN MESSAGE: Remote command execution failed: bash: /usr/local/nagios/libexec/check_mem: No such file or directory`

It is worth evaluating whether we could have a "critical eyecatcher" phrase of some sort that would be added into all such lines posted to slack (Possibly all the ones with the flame or ? prefix)

FYI @steelhead31 another requirement for you :-)