Allow using the default `log` alert for both up and down alerts using
Go's templating conditionals. Following this example can do away with
the need for an up and down version of every alert.
This makes the configuration more similar to Minitor-py and
docker-compose. If a string is passed, it will be executed in a shell.
If an array is passed, it will be executed in as a command directly.
This breaks compatiblity with previous versions of Minitor-go, but
closer to compatiblity with Minitor-py.
