(feature request) Nag the user when they have mrtest packages installed

Sometimes if the user forgets to remove mrtest package, they might end up with broken OS (i.e. if they were testing initfs changes and a later update caused a conflict, apk update might result in an unbootatbe system.)

To make sure user always knows about the installed mrtest applications, let's install a "nag" post-commit hook that would run at the very end of apk install/update and show a warning message.

A simple example of such hook:

$ cat /etc/apk/commit_hooks.d/mrtest_installed 
#!/bin/sh

case "$1" in
	pre-commit)
		;;
	post-commit)
		echo "*"
		echo "* You have some packages installed via mrtest!"
		echo "*"

	*)
		echo "$0: Un-handled action: $1"
		;;
esac